log->l_cilp->xc_ctx->sequence = 1;
}
+static inline int
+xlog_cil_iovec_space(
+ uint niovecs)
+{
+ return round_up((sizeof(struct xfs_log_vec) +
+ niovecs * sizeof(struct xfs_log_iovec)),
+ sizeof(uint64_t));
+}
+
+/*
+ * Allocate or pin log vector buffers for CIL insertion.
+ *
+ * The CIL currently uses disposable buffers for copying a snapshot of the
+ * modified items into the log during a push. The biggest problem with this is
+ * the requirement to allocate the disposable buffer during the commit if:
+ * a) does not exist; or
+ * b) it is too small
+ *
+ * If we do this allocation within xlog_cil_insert_format_items(), it is done
+ * under the xc_ctx_lock, which means that a CIL push cannot occur during
+ * the memory allocation. This means that we have a potential deadlock situation
+ * under low memory conditions when we have lots of dirty metadata pinned in
+ * the CIL and we need a CIL commit to occur to free memory.
+ *
+ * To avoid this, we need to move the memory allocation outside the
+ * xc_ctx_lock, but because the log vector buffers are disposable, that opens
+ * up a TOCTOU race condition w.r.t. the CIL committing and removing the log
+ * vector buffers between the check and the formatting of the item into the
+ * log vector buffer within the xc_ctx_lock.
+ *
+ * Because the log vector buffer needs to be unchanged during the CIL push
+ * process, we cannot share the buffer between the transaction commit (which
+ * modifies the buffer) and the CIL push context that is writing the changes
+ * into the log. This means skipping preallocation of buffer space is
+ * unreliable, but we most definitely do not want to be allocating and freeing
+ * buffers unnecessarily during commits when overwrites can be done safely.
+ *
+ * The simplest solution to this problem is to allocate a shadow buffer when a
+ * log item is committed for the second time, and then to only use this buffer
+ * if necessary. The buffer can remain attached to the log item until such time
+ * it is needed, and this is the buffer that is reallocated to match the size of
+ * the incoming modification. Then during the formatting of the item we can swap
+ * the active buffer with the new one if we can't reuse the existing buffer. We
+ * don't free the old buffer as it may be reused on the next modification if
+ * it's size is right, otherwise we'll free and reallocate it at that point.
+ *
+ * This function builds a vector for the changes in each log item in the
+ * transaction. It then works out the length of the buffer needed for each log
+ * item, allocates them and attaches the vector to the log item in preparation
+ * for the formatting step which occurs under the xc_ctx_lock.
+ *
+ * While this means the memory footprint goes up, it avoids the repeated
+ * alloc/free pattern that repeated modifications of an item would otherwise
+ * cause, and hence minimises the CPU overhead of such behaviour.
+ */
+static void
+xlog_cil_alloc_shadow_bufs(
+ struct xlog *log,
+ struct xfs_trans *tp)
+{
+ struct xfs_log_item_desc *lidp;
+
+ list_for_each_entry(lidp, &tp->t_items, lid_trans) {
+ struct xfs_log_item *lip = lidp->lid_item;
+ struct xfs_log_vec *lv;
+ int niovecs = 0;
+ int nbytes = 0;
+ int buf_size;
+ bool ordered = false;
+
+ /* Skip items which aren't dirty in this transaction. */
+ if (!(lidp->lid_flags & XFS_LID_DIRTY))
+ continue;
+
+ /* get number of vecs and size of data to be stored */
+ lip->li_ops->iop_size(lip, &niovecs, &nbytes);
+
+ /*
+ * Ordered items need to be tracked but we do not wish to write
+ * them. We need a logvec to track the object, but we do not
+ * need an iovec or buffer to be allocated for copying data.
+ */
+ if (niovecs == XFS_LOG_VEC_ORDERED) {
+ ordered = true;
+ niovecs = 0;
+ nbytes = 0;
+ }
+
+ /*
+ * We 64-bit align the length of each iovec so that the start
+ * of the next one is naturally aligned. We'll need to
+ * account for that slack space here. Then round nbytes up
+ * to 64-bit alignment so that the initial buffer alignment is
+ * easy to calculate and verify.
+ */
+ nbytes += niovecs * sizeof(uint64_t);
+ nbytes = round_up(nbytes, sizeof(uint64_t));
+
+ /*
+ * The data buffer needs to start 64-bit aligned, so round up
+ * that space to ensure we can align it appropriately and not
+ * overrun the buffer.
+ */
+ buf_size = nbytes + xlog_cil_iovec_space(niovecs);
+
+ /*
+ * if we have no shadow buffer, or it is too small, we need to
+ * reallocate it.
+ */
+ if (!lip->li_lv_shadow ||
+ buf_size > lip->li_lv_shadow->lv_size) {
+
+ /*
+ * We free and allocate here as a realloc would copy
+ * unecessary data. We don't use kmem_zalloc() for the
+ * same reason - we don't need to zero the data area in
+ * the buffer, only the log vector header and the iovec
+ * storage.
+ */
+ kmem_free(lip->li_lv_shadow);
+
+ lv = kmem_alloc(buf_size, KM_SLEEP|KM_NOFS);
+ memset(lv, 0, xlog_cil_iovec_space(niovecs));
+
+ lv->lv_item = lip;
+ lv->lv_size = buf_size;
+ if (ordered)
+ lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
+ else
+ lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
+ lip->li_lv_shadow = lv;
+ } else {
+ /* same or smaller, optimise common overwrite case */
+ lv = lip->li_lv_shadow;
+ if (ordered)
+ lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
+ else
+ lv->lv_buf_len = 0;
+ lv->lv_bytes = 0;
+ lv->lv_next = NULL;
+ }
+
+ /* Ensure the lv is set up according to ->iop_size */
+ lv->lv_niovecs = niovecs;
+
+ /* The allocated data region lies beyond the iovec region */
+ lv->lv_buf = (char *)lv + xlog_cil_iovec_space(niovecs);
+ }
+
+}
+
/*
* Prepare the log item for insertion into the CIL. Calculate the difference in
* log space and vectors it will consume, and if it is a new item pin it as
/*
* If there is no old LV, this is the first time we've seen the item in
* this CIL context and so we need to pin it. If we are replacing the
- * old_lv, then remove the space it accounts for and free it.
+ * old_lv, then remove the space it accounts for and make it the shadow
+ * buffer for later freeing. In both cases we are now switching to the
+ * shadow buffer, so update the the pointer to it appropriately.
*/
- if (!old_lv)
+ if (!old_lv) {
lv->lv_item->li_ops->iop_pin(lv->lv_item);
- else if (old_lv != lv) {
+ lv->lv_item->li_lv_shadow = NULL;
+ } else if (old_lv != lv) {
ASSERT(lv->lv_buf_len != XFS_LOG_VEC_ORDERED);
*diff_len -= old_lv->lv_bytes;
*diff_iovecs -= old_lv->lv_niovecs;
- kmem_free(old_lv);
+ lv->lv_item->li_lv_shadow = old_lv;
}
/* attach new log vector to log item */
* write it out asynchronously without needing to relock the object that was
* modified at the time it gets written into the iclog.
*
- * This function builds a vector for the changes in each log item in the
- * transaction. It then works out the length of the buffer needed for each log
- * item, allocates them and formats the vector for the item into the buffer.
- * The buffer is then attached to the log item are then inserted into the
- * Committed Item List for tracking until the next checkpoint is written out.
+ * This function takes the prepared log vectors attached to each log item, and
+ * formats the changes into the log vector buffer. The buffer it uses is
+ * dependent on the current state of the vector in the CIL - the shadow lv is
+ * guaranteed to be large enough for the current modification, but we will only
+ * use that if we can't reuse the existing lv. If we can't reuse the existing
+ * lv, then simple swap it out for the shadow lv. We don't free it - that is
+ * done lazily either by th enext modification or the freeing of the log item.
*
* We don't set up region headers during this process; we simply copy the
* regions into the flat buffer. We can do this because we still have to do a
list_for_each_entry(lidp, &tp->t_items, lid_trans) {
struct xfs_log_item *lip = lidp->lid_item;
struct xfs_log_vec *lv;
- struct xfs_log_vec *old_lv;
- int niovecs = 0;
- int nbytes = 0;
- int buf_size;
+ struct xfs_log_vec *old_lv = NULL;
+ struct xfs_log_vec *shadow;
bool ordered = false;
/* Skip items which aren't dirty in this transaction. */
if (!(lidp->lid_flags & XFS_LID_DIRTY))
continue;
- /* get number of vecs and size of data to be stored */
- lip->li_ops->iop_size(lip, &niovecs, &nbytes);
-
- /* Skip items that do not have any vectors for writing */
- if (!niovecs)
- continue;
-
/*
- * Ordered items need to be tracked but we do not wish to write
- * them. We need a logvec to track the object, but we do not
- * need an iovec or buffer to be allocated for copying data.
+ * The formatting size information is already attached to
+ * the shadow lv on the log item.
*/
- if (niovecs == XFS_LOG_VEC_ORDERED) {
+ shadow = lip->li_lv_shadow;
+ if (shadow->lv_buf_len == XFS_LOG_VEC_ORDERED)
ordered = true;
- niovecs = 0;
- nbytes = 0;
- }
- /*
- * We 64-bit align the length of each iovec so that the start
- * of the next one is naturally aligned. We'll need to
- * account for that slack space here. Then round nbytes up
- * to 64-bit alignment so that the initial buffer alignment is
- * easy to calculate and verify.
- */
- nbytes += niovecs * sizeof(uint64_t);
- nbytes = round_up(nbytes, sizeof(uint64_t));
-
- /* grab the old item if it exists for reservation accounting */
- old_lv = lip->li_lv;
-
- /*
- * The data buffer needs to start 64-bit aligned, so round up
- * that space to ensure we can align it appropriately and not
- * overrun the buffer.
- */
- buf_size = nbytes +
- round_up((sizeof(struct xfs_log_vec) +
- niovecs * sizeof(struct xfs_log_iovec)),
- sizeof(uint64_t));
+ /* Skip items that do not have any vectors for writing */
+ if (!shadow->lv_niovecs && !ordered)
+ continue;
/* compare to existing item size */
- if (lip->li_lv && buf_size <= lip->li_lv->lv_size) {
+ old_lv = lip->li_lv;
+ if (lip->li_lv && shadow->lv_size <= lip->li_lv->lv_size) {
/* same or smaller, optimise common overwrite case */
lv = lip->li_lv;
lv->lv_next = NULL;
*/
*diff_iovecs -= lv->lv_niovecs;
*diff_len -= lv->lv_bytes;
+
+ /* Ensure the lv is set up according to ->iop_size */
+ lv->lv_niovecs = shadow->lv_niovecs;
+
+ /* reset the lv buffer information for new formatting */
+ lv->lv_buf_len = 0;
+ lv->lv_bytes = 0;
+ lv->lv_buf = (char *)lv +
+ xlog_cil_iovec_space(lv->lv_niovecs);
} else {
- /* allocate new data chunk */
- lv = kmem_zalloc(buf_size, KM_SLEEP|KM_NOFS);
+ /* switch to shadow buffer! */
+ lv = shadow;
lv->lv_item = lip;
- lv->lv_size = buf_size;
if (ordered) {
/* track as an ordered logvec */
ASSERT(lip->li_lv == NULL);
- lv->lv_buf_len = XFS_LOG_VEC_ORDERED;
goto insert;
}
- lv->lv_iovecp = (struct xfs_log_iovec *)&lv[1];
}
- /* Ensure the lv is set up according to ->iop_size */
- lv->lv_niovecs = niovecs;
-
- /* The allocated data region lies beyond the iovec region */
- lv->lv_buf_len = 0;
- lv->lv_bytes = 0;
- lv->lv_buf = (char *)lv + buf_size - nbytes;
ASSERT(IS_ALIGNED((unsigned long)lv->lv_buf, sizeof(uint64_t)));
-
lip->li_ops->iop_format(lip, lv);
insert:
- ASSERT(lv->lv_buf_len <= nbytes);
xfs_cil_prepare_item(log, lv, old_lv, diff_len, diff_iovecs);
}
}
struct xlog *log = mp->m_log;
struct xfs_cil *cil = log->l_cilp;
+ /*
+ * Do all necessary memory allocation before we lock the CIL.
+ * This ensures the allocation does not deadlock with a CIL
+ * push in memory reclaim (e.g. from kswapd).
+ */
+ xlog_cil_alloc_shadow_bufs(log, tp);
+
/* lock out background commit */
down_read(&cil->xc_ctx_lock);
projects / GitHub / LineageOS / android_kernel_motorola_exynos9610.git / commitdiff