From: NeilBrown Date: Fri, 20 May 2016 23:58:53 +0000 (-0700) Subject: MM: increase safety margin provided by PF_LESS_THROTTLE X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=a53eaff8c1192bb5bdfda5deb484bc8f415c5dfd;p=GitHub%2Fmoto-9609%2Fandroid_kernel_motorola_exynos9610.git MM: increase safety margin provided by PF_LESS_THROTTLE When nfsd is exporting a filesystem over NFS which is then NFS-mounted on the local machine there is a risk of deadlock. This happens when there are lots of dirty pages in the NFS filesystem and they cause NFSD to be throttled, either in throttle_vm_writeout() or in balance_dirty_pages(). To avoid this problem the PF_LESS_THROTTLE flag is set for NFSD threads and it provides a 25% increase to the limits that affect NFSD. Any process writing to an NFS filesystem will be throttled well before the number of dirty NFS pages reaches the limit imposed on NFSD, so NFSD will not deadlock on pages that it needs to write out. At least it shouldn't. All processes are allowed a small excess margin to avoid performing too many calculations: ratelimit_pages. ratelimit_pages is set so that if a thread on every CPU uses the entire margin, the total will only go 3% over the limit, and this is much less than the 25% bonus that PF_LESS_THROTTLE provides, so this margin shouldn't be a problem. But it is. The "total memory" that these 3% and 25% are calculated against are not really total memory but are "global_dirtyable_memory()" which doesn't include anonymous memory, just free memory and page-cache memory. The "ratelimit_pages" number is based on whatever the global_dirtyable_memory was on the last CPU hot-plug, which might not be what you expect, but is probably close to the total freeable memory. The throttle threshold uses the global_dirtable_memory at the moment when the throttling happens, which could be much less than at the last CPU hotplug. So if lots of anonymous memory has been allocated, thus pushing out lots of page-cache pages, then NFSD might end up being throttled due to dirty NFS pages because the "25%" bonus it gets is calculated against a rather small amount of dirtyable memory, while the "3%" margin that other processes are allowed to dirty without penalty is calculated against a much larger number. To remove this possibility of deadlock we need to make sure that the margin granted to PF_LESS_THROTTLE exceeds that rate-limit margin. Simply adding ratelimit_pages isn't enough as that should be multiplied by the number of cpus. So add "global_wb_domain.dirty_limit / 32" as that more accurately reflects the current total over-shoot margin. This ensures that the number of dirty NFS pages never gets so high that nfsd will be throttled waiting for them to be written. Link: http://lkml.kernel.org/r/87futgowwv.fsf@notabene.neil.brown.name Signed-off-by: NeilBrown Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 3b88795ab46e..b9956fdee8f5 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -411,8 +411,8 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc) bg_thresh = thresh / 2; tsk = current; if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) { - bg_thresh += bg_thresh / 4; - thresh += thresh / 4; + bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32; + thresh += thresh / 4 + global_wb_domain.dirty_limit / 32; } dtc->thresh = thresh; dtc->bg_thresh = bg_thresh;