From: Vlastimil Babka Date: Thu, 14 Jan 2016 23:19:17 +0000 (-0800) Subject: mm, proc: account for shmem swap in /proc/pid/smaps X-Git-Url: http://git.cdn.openwrt.org/?a=commitdiff_plain;h=c261e7d94f0dd33a34b6cf98686e8b9699b62340;p=openwrt%2Fstaging%2Fblogic.git mm, proc: account for shmem swap in /proc/pid/smaps Currently, /proc/pid/smaps will always show "Swap: 0 kB" for shmem-backed mappings, even if the mapped portion does contain pages that were swapped out. This is because unlike private anonymous mappings, shmem does not change pte to swap entry, but pte_none when swapping the page out. In the smaps page walk, such page thus looks like it was never faulted in. This patch changes smaps_pte_entry() to determine the swap status for such pte_none entries for shmem mappings, similarly to how mincore_page() does it. Swapped out shmem pages are thus accounted for. For private mappings of tmpfs files that COWed some of the pages, swaped out status of the original shmem pages is naturally ignored. If some of the private copies was also swapped out, they are accounted via their page table swap entries, so the resulting reported swap usage is then a sum of both swapped out private copies, and swapped out shmem pages that were not COWed. No double accounting can thus happen. The accounting is arguably still not as precise as for private anonymous mappings, since now we will count also pages that the process in question never accessed, but another process populated them and then let them become swapped out. I believe it is still less confusing and subtle than not showing any swap usage by shmem mappings at all. Swapped out counter might of interest of users who would like to prevent from future swapins during performance critical operation and pre-fault them at their convenience. Especially for larger swapped out regions the cost of swapin is much higher than a fresh page allocation. So a differentiation between pte_none vs. swapped out is important for those usecases. One downside of this patch is that it makes /proc/pid/smaps more expensive for shmem mappings, as we consult the radix tree for each pte_none entry, so the overal complexity is O(n*log(n)). I have measured this on a process that creates a 2GB mapping and dirties single pages with a stride of 2MB, and time how long does it take to cat /proc/pid/smaps of this process 100 times. Private anonymous mapping: real 0m0.949s user 0m0.116s sys 0m0.348s Mapping of a /dev/shm/file: real 0m3.831s user 0m0.180s sys 0m3.212s The difference is rather substantial, so the next patch will reduce the cost for shared or read-only mappings. In a less controlled experiment, I've gathered pids of processes on my desktop that have either '/dev/shm/*' or 'SYSV*' in smaps. This included the Chrome browser and some KDE processes. Again, I've run cat /proc/pid/smaps on each 100 times. Before this patch: real 0m9.050s user 0m0.518s sys 0m8.066s After this patch: real 0m9.221s user 0m0.541s sys 0m8.187s This suggests low impact on average systems. Note that this patch doesn't attempt to adjust the SwapPss field for shmem mappings, which would need extra work to determine who else could have the pages mapped. Thus the value stays zero except for COWed swapped out pages in a shmem mapping, which are accounted as usual. Signed-off-by: Vlastimil Babka Acked-by: Konstantin Khlebnikov Acked-by: Jerome Marchand Acked-by: Michal Hocko Cc: Hugh Dickins Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt index 9f13b6e28676..fdeb5b33349f 100644 --- a/Documentation/filesystems/proc.txt +++ b/Documentation/filesystems/proc.txt @@ -460,7 +460,10 @@ and a page is modified, the file page is replaced by a private anonymous copy. hugetlbfs page which is *not* counted in "RSS" or "PSS" field for historical reasons. And these are not included in {Shared,Private}_{Clean,Dirty} field. "Swap" shows how much would-be-anonymous memory is also used, but out on swap. -"SwapPss" shows proportional swap share of this mapping. +For shmem mappings, "Swap" includes also the size of the mapped (and not +replaced by copy-on-write) part of the underlying shmem object out on swap. +"SwapPss" shows proportional swap share of this mapping. Unlike "Swap", this +does not take into account swapped out page of underlying shmem objects. "Locked" indicates whether the mapping is locked in memory or not. "VmFlags" field deserves a separate description. This member represents the kernel diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 187b3b5f242e..85ef60fdf2c0 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -451,6 +451,7 @@ struct mem_size_stats { unsigned long private_hugetlb; u64 pss; u64 swap_pss; + bool check_shmem_swap; }; static void smaps_account(struct mem_size_stats *mss, struct page *page, @@ -485,6 +486,45 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page, } } +#ifdef CONFIG_SHMEM +static unsigned long smaps_shmem_swap(struct vm_area_struct *vma, + unsigned long addr) +{ + struct page *page; + + page = find_get_entry(vma->vm_file->f_mapping, + linear_page_index(vma, addr)); + if (!page) + return 0; + + if (radix_tree_exceptional_entry(page)) + return PAGE_SIZE; + + page_cache_release(page); + return 0; + +} + +static int smaps_pte_hole(unsigned long addr, unsigned long end, + struct mm_walk *walk) +{ + struct mem_size_stats *mss = walk->private; + + while (addr < end) { + mss->swap += smaps_shmem_swap(walk->vma, addr); + addr += PAGE_SIZE; + } + + return 0; +} +#else +static unsigned long smaps_shmem_swap(struct vm_area_struct *vma, + unsigned long addr) +{ + return 0; +} +#endif + static void smaps_pte_entry(pte_t *pte, unsigned long addr, struct mm_walk *walk) { @@ -512,6 +552,9 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr, } } else if (is_migration_entry(swpent)) page = migration_entry_to_page(swpent); + } else if (unlikely(IS_ENABLED(CONFIG_SHMEM) && mss->check_shmem_swap + && pte_none(*pte))) { + mss->swap += smaps_shmem_swap(vma, addr); } if (!page) @@ -671,6 +714,14 @@ static int show_smap(struct seq_file *m, void *v, int is_pid) }; memset(&mss, 0, sizeof mss); + +#ifdef CONFIG_SHMEM + if (vma->vm_file && shmem_mapping(vma->vm_file->f_mapping)) { + mss.check_shmem_swap = true; + smaps_walk.pte_hole = smaps_pte_hole; + } +#endif + /* mmap_sem is held in m_start */ walk_page_vma(vma, &smaps_walk);