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Date: Wed, 29 Jun 2011 14:09:03 +0200
From: Michal Hocko <mhocko@...e.cz>
To: akpm@...ux-foundation.org
Cc: kamezawa.hiroyu@...fujitsu.com, hannes@...xchg.org, lvml@....de,
nishimura@....nes.nec.co.jp, yinghan@...gle.com,
LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: + mm-preallocate-page-before-lock_page-at-filemap-cow.patch
added to -mm tree
Shouldn't we discuss unlock, charge, unlock&recheck_mapping approach
as well before this gets merged? There are some concerns expressed
during discussion - https://lkml.org/lkml/2011/6/23/135.
The full thread is at https://lkml.org/lkml/2011/6/22/163
I do not mind which bugfix is used in the end I would just be happier if
they would get some more review.
On Tue 28-06-11 17:17:20, Andrew Morton wrote:
> ------------------------------------------------------
> Subject: mm: preallocate page before lock_page() at filemap COW
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> Currently we are keeping faulted page locked throughout whole __do_fault
> call (except for page_mkwrite code path) after calling file system's fault
> code. If we do early COW, we allocate a new page which has to be charged
> for a memcg (mem_cgroup_newpage_charge).
>
> This function, however, might block for unbounded amount of time if memcg
> oom killer is disabled or fork-bomb is running because the only way out of
> the OOM situation is either an external event or OOM-situation fix.
>
> In the end we are keeping the faulted page locked and blocking other
> processes from faulting it in which is not good at all because we are
> basically punishing potentially an unrelated process for OOM condition in
> a different group (I have seen stuck system because of ld-2.11.1.so being
> locked).
>
> We can do test easily.
>
> % cgcreate -g memory:A
> % cgset -r memory.limit_in_bytes=64M A
> % cgset -r memory.memsw.limit_in_bytes=64M A
> % cd kernel_dir; cgexec -g memory:A make -j
>
> Then, the whole system will live-locked until you kill 'make -j'
> by hands (or push reboot...) This is because some important page in a
> a shared library are locked.
>
> Considering again, the new page is not necessary to be allocated
> with lock_page() held. And usual page allocation may dive into
> long memory reclaim loop with holding lock_page() and can cause
> very long latency.
>
> There are 3 ways.
> 1. do allocation/charge before lock_page()
> Pros. - simple and can handle page allocation in the same manner.
> This will reduce holding time of lock_page() in general.
> Cons. - we do page allocation even if ->fault() returns error.
>
> 2. do charge after unlock_page(). Even if charge fails, it's just OOM.
> Pros. - no impact to non-memcg path.
> Cons. - implemenation requires special cares of LRU and we need to modify
> page_add_new_anon_rmap()...
>
> 3. do unlock->charge->lock again method.
> Pros. - no impact to non-memcg path.
> Cons. - This may kill LOCK_PAGE_RETRY optimization. We need to release
> lock and get it again...
>
> This patch moves "charge" and memory allocation for COW page
> before lock_page(). Then, we can avoid scanning LRU with holding
> a lock on a page and latency under lock_page() will be reduced.
>
> Then, above livelock disappears.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Reported-by: Lutz Vieweg <lvml@....de>
> Original-idea-by: Michal Hocko <mhocko@...e.cz>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Ying Han <yinghan@...gle.com>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
>
> mm/memory.c | 56 ++++++++++++++++++++++++++++++--------------------
> 1 file changed, 34 insertions(+), 22 deletions(-)
>
> diff -puN mm/memory.c~mm-preallocate-page-before-lock_page-at-filemap-cow mm/memory.c
> --- a/mm/memory.c~mm-preallocate-page-before-lock_page-at-filemap-cow
> +++ a/mm/memory.c
> @@ -3104,14 +3104,34 @@ static int __do_fault(struct mm_struct *
> pte_t *page_table;
> spinlock_t *ptl;
> struct page *page;
> + struct page *cow_page;
> pte_t entry;
> int anon = 0;
> - int charged = 0;
> struct page *dirty_page = NULL;
> struct vm_fault vmf;
> int ret;
> int page_mkwrite = 0;
>
> + /*
> + * If we do COW later, allocate page befor taking lock_page()
> + * on the file cache page. This will reduce lock holding time.
> + */
> + if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
> +
> + if (unlikely(anon_vma_prepare(vma)))
> + return VM_FAULT_OOM;
> +
> + cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, address);
> + if (!cow_page)
> + return VM_FAULT_OOM;
> +
> + if (mem_cgroup_newpage_charge(cow_page, mm, GFP_KERNEL)) {
> + page_cache_release(cow_page);
> + return VM_FAULT_OOM;
> + }
> + } else
> + cow_page = NULL;
> +
> vmf.virtual_address = (void __user *)(address & PAGE_MASK);
> vmf.pgoff = pgoff;
> vmf.flags = flags;
> @@ -3120,12 +3140,13 @@ static int __do_fault(struct mm_struct *
> ret = vma->vm_ops->fault(vma, &vmf);
> if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE |
> VM_FAULT_RETRY)))
> - return ret;
> + goto uncharge_out;
>
> if (unlikely(PageHWPoison(vmf.page))) {
> if (ret & VM_FAULT_LOCKED)
> unlock_page(vmf.page);
> - return VM_FAULT_HWPOISON;
> + ret = VM_FAULT_HWPOISON;
> + goto uncharge_out;
> }
>
> /*
> @@ -3143,23 +3164,8 @@ static int __do_fault(struct mm_struct *
> page = vmf.page;
> if (flags & FAULT_FLAG_WRITE) {
> if (!(vma->vm_flags & VM_SHARED)) {
> + page = cow_page;
> anon = 1;
> - if (unlikely(anon_vma_prepare(vma))) {
> - ret = VM_FAULT_OOM;
> - goto out;
> - }
> - page = alloc_page_vma(GFP_HIGHUSER_MOVABLE,
> - vma, address);
> - if (!page) {
> - ret = VM_FAULT_OOM;
> - goto out;
> - }
> - if (mem_cgroup_newpage_charge(page, mm, GFP_KERNEL)) {
> - ret = VM_FAULT_OOM;
> - page_cache_release(page);
> - goto out;
> - }
> - charged = 1;
> copy_user_highpage(page, vmf.page, address, vma);
> __SetPageUptodate(page);
> } else {
> @@ -3228,8 +3234,8 @@ static int __do_fault(struct mm_struct *
> /* no need to invalidate: a not-present page won't be cached */
> update_mmu_cache(vma, address, page_table);
> } else {
> - if (charged)
> - mem_cgroup_uncharge_page(page);
> + if (cow_page)
> + mem_cgroup_uncharge_page(cow_page);
> if (anon)
> page_cache_release(page);
> else
> @@ -3238,7 +3244,6 @@ static int __do_fault(struct mm_struct *
>
> pte_unmap_unlock(page_table, ptl);
>
> -out:
> if (dirty_page) {
> struct address_space *mapping = page->mapping;
>
> @@ -3268,6 +3273,13 @@ out:
> unwritable_page:
> page_cache_release(page);
> return ret;
> +uncharge_out:
> + /* fs's fault handler get error */
> + if (cow_page) {
> + mem_cgroup_uncharge_page(cow_page);
> + page_cache_release(cow_page);
> + }
> + return ret;
> }
>
> static int do_linear_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> _
>
> Patches currently in -mm which might be from kamezawa.hiroyu@...fujitsu.com are
>
> memcg-fix-reclaimable-lru-check-in-memcg.patch
> memcg-fix-reclaimable-lru-check-in-memcg-checkpatch-fixes.patch
> memcg-fix-reclaimable-lru-check-in-memcg-fix.patch
> memcg-fix-numa-scan-information-update-to-be-triggered-by-memory-event.patch
> memcg-fix-numa-scan-information-update-to-be-triggered-by-memory-event-fix.patch
> mm-preallocate-page-before-lock_page-at-filemap-cow.patch
> mm-preallocate-page-before-lock_page-at-filemap-cow-fix.patch
> mm-page_cgroupc-simplify-code-by-using-section_align_up-and-section_align_down-macros.patch
> memcg-do-not-expose-uninitialized-mem_cgroup_per_node-to-world.patch
>
--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic
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