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@xxxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxxx> > Reported-by: Lutz Vieweg <lvml@xxxxxx> > Original-idea-by: Michal Hocko <mhocko@xxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxx> > Cc: Ying Han <yinghan@xxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > --- > > 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@xxxxxxxxxxxxxx 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. 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