From 08291429cfa6258c4cd95d8833beb40f828b194e Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Tue, 16 Oct 2007 01:24:59 -0700 Subject: [PATCH] mm: fix pagecache write deadlocks Modify the core write() code so that it won't take a pagefault while holding a lock on the pagecache page. There are a number of different deadlocks possible if we try to do such a thing: 1. generic_buffered_write 2. lock_page 3. prepare_write 4. unlock_page+vmtruncate 5. copy_from_user 6. mmap_sem(r) 7. handle_mm_fault 8. lock_page (filemap_nopage) 9. commit_write 10. unlock_page a. sys_munmap / sys_mlock / others b. mmap_sem(w) c. make_pages_present d. get_user_pages e. handle_mm_fault f. lock_page (filemap_nopage) 2,8 - recursive deadlock if page is same 2,8;2,8 - ABBA deadlock is page is different 2,6;b,f - ABBA deadlock if page is same The solution is as follows: 1. If we find the destination page is uptodate, continue as normal, but use atomic usercopies which do not take pagefaults and do not zero the uncopied tail of the destination. The destination is already uptodate, so we can commit_write the full length even if there was a partial copy: it does not matter that the tail was not modified, because if it is dirtied and written back to disk it will not cause any problems (uptodate *means* that the destination page is as new or newer than the copy on disk). 1a. The above requires that fault_in_pages_readable correctly returns access information, because atomic usercopies cannot distinguish between non-present pages in a readable mapping, from lack of a readable mapping. 2. If we find the destination page is non uptodate, unlock it (this could be made slightly more optimal), then allocate a temporary page to copy the source data into. Relock the destination page and continue with the copy. However, instead of a usercopy (which might take a fault), copy the data from the pinned temporary page via the kernel address space. (also, rename maxlen to seglen, because it was confusing) This increases the CPU/memory copy cost by almost 50% on the affected workloads. That will be solved by introducing a new set of pagecache write aops in a subsequent patch. Signed-off-by: Nick Piggin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/pagemap.h | 11 +++- mm/filemap.c | 122 +++++++++++++++++++++++++++++++++------- 2 files changed, 112 insertions(+), 21 deletions(-) diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h index 862fc07dc6c0..8f1e390fd71b 100644 --- a/include/linux/pagemap.h +++ b/include/linux/pagemap.h @@ -219,6 +219,9 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size) { int ret; + if (unlikely(size == 0)) + return 0; + /* * Writing zeroes into userspace here is OK, because we know that if * the zero gets there, we'll be overwriting it. @@ -238,19 +241,23 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size) return ret; } -static inline void fault_in_pages_readable(const char __user *uaddr, int size) +static inline int fault_in_pages_readable(const char __user *uaddr, int size) { volatile char c; int ret; + if (unlikely(size == 0)) + return 0; + ret = __get_user(c, uaddr); if (ret == 0) { const char __user *end = uaddr + size - 1; if (((unsigned long)uaddr & PAGE_MASK) != ((unsigned long)end & PAGE_MASK)) - __get_user(c, end); + ret = __get_user(c, end); } + return ret; } #endif /* _LINUX_PAGEMAP_H */ diff --git a/mm/filemap.c b/mm/filemap.c index c59d5b3cd99a..557fd887254f 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1826,11 +1826,12 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, filemap_set_next_iovec(&cur_iov, nr_segs, &iov_offset, written); do { + struct page *src_page; struct page *page; pgoff_t index; /* Pagecache index for current page */ unsigned long offset; /* Offset into pagecache page */ - unsigned long maxlen; /* Bytes remaining in current iovec */ - size_t bytes; /* Bytes to write to page */ + unsigned long seglen; /* Bytes remaining in current iovec */ + unsigned long bytes; /* Bytes to write to page */ size_t copied; /* Bytes copied from user */ buf = cur_iov->iov_base + iov_offset; @@ -1840,20 +1841,30 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, if (bytes > count) bytes = count; - maxlen = cur_iov->iov_len - iov_offset; - if (maxlen > bytes) - maxlen = bytes; + /* + * a non-NULL src_page indicates that we're doing the + * copy via get_user_pages and kmap. + */ + src_page = NULL; + + seglen = cur_iov->iov_len - iov_offset; + if (seglen > bytes) + seglen = bytes; -#ifndef CONFIG_DEBUG_VM /* * Bring in the user page that we will copy from _first_. * Otherwise there's a nasty deadlock on copying from the * same page as we're writing to, without it being marked * up-to-date. + * + * Not only is this an optimisation, but it is also required + * to check that the address is actually valid, when atomic + * usercopies are used, below. */ - fault_in_pages_readable(buf, maxlen); -#endif - + if (unlikely(fault_in_pages_readable(buf, seglen))) { + status = -EFAULT; + break; + } page = __grab_cache_page(mapping, index); if (!page) { @@ -1861,32 +1872,104 @@ generic_file_buffered_write(struct kiocb *iocb, const struct iovec *iov, break; } + /* + * non-uptodate pages cannot cope with short copies, and we + * cannot take a pagefault with the destination page locked. + * So pin the source page to copy it. + */ + if (!PageUptodate(page)) { + unlock_page(page); + + src_page = alloc_page(GFP_KERNEL); + if (!src_page) { + page_cache_release(page); + status = -ENOMEM; + break; + } + + /* + * Cannot get_user_pages with a page locked for the + * same reason as we can't take a page fault with a + * page locked (as explained below). + */ + copied = filemap_copy_from_user(src_page, offset, + cur_iov, nr_segs, iov_offset, bytes); + if (unlikely(copied == 0)) { + status = -EFAULT; + page_cache_release(page); + page_cache_release(src_page); + break; + } + bytes = copied; + + lock_page(page); + /* + * Can't handle the page going uptodate here, because + * that means we would use non-atomic usercopies, which + * zero out the tail of the page, which can cause + * zeroes to become transiently visible. We could just + * use a non-zeroing copy, but the APIs aren't too + * consistent. + */ + if (unlikely(!page->mapping || PageUptodate(page))) { + unlock_page(page); + page_cache_release(page); + page_cache_release(src_page); + continue; + } + + } + status = a_ops->prepare_write(file, page, offset, offset+bytes); if (unlikely(status)) goto fs_write_aop_error; - copied = filemap_copy_from_user(page, offset, + if (!src_page) { + /* + * Must not enter the pagefault handler here, because + * we hold the page lock, so we might recursively + * deadlock on the same lock, or get an ABBA deadlock + * against a different lock, or against the mmap_sem + * (which nests outside the page lock). So increment + * preempt count, and use _atomic usercopies. + * + * The page is uptodate so we are OK to encounter a + * short copy: if unmodified parts of the page are + * marked dirty and written out to disk, it doesn't + * really matter. + */ + pagefault_disable(); + copied = filemap_copy_from_user_atomic(page, offset, cur_iov, nr_segs, iov_offset, bytes); + pagefault_enable(); + } else { + void *src, *dst; + src = kmap_atomic(src_page, KM_USER0); + dst = kmap_atomic(page, KM_USER1); + memcpy(dst + offset, src + offset, bytes); + kunmap_atomic(dst, KM_USER1); + kunmap_atomic(src, KM_USER0); + copied = bytes; + } flush_dcache_page(page); status = a_ops->commit_write(file, page, offset, offset+bytes); if (unlikely(status < 0 || status == AOP_TRUNCATED_PAGE)) goto fs_write_aop_error; - if (unlikely(copied != bytes)) { - status = -EFAULT; - goto fs_write_aop_error; - } if (unlikely(status > 0)) /* filesystem did partial write */ - copied = status; + copied = min_t(size_t, copied, status); + + unlock_page(page); + mark_page_accessed(page); + page_cache_release(page); + if (src_page) + page_cache_release(src_page); written += copied; count -= copied; pos += copied; filemap_set_next_iovec(&cur_iov, nr_segs, &iov_offset, copied); - unlock_page(page); - mark_page_accessed(page); - page_cache_release(page); balance_dirty_pages_ratelimited(mapping); cond_resched(); continue; @@ -1895,6 +1978,8 @@ fs_write_aop_error: if (status != AOP_TRUNCATED_PAGE) unlock_page(page); page_cache_release(page); + if (src_page) + page_cache_release(src_page); /* * prepare_write() may have instantiated a few blocks @@ -1907,7 +1992,6 @@ fs_write_aop_error: continue; else break; - } while (count); *ppos = pos; -- 2.20.1