Messages in this thread | | | Subject | Re: [RFC PATCH 1/1] pagemap: report swap location for shared pages | From | David Hildenbrand <> | Date | Wed, 14 Jul 2021 18:30:05 +0200 |
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On 14.07.21 18:24, David Hildenbrand wrote: > On 14.07.21 18:08, Peter Xu wrote: >> On Wed, Jul 14, 2021 at 03:24:26PM +0000, Tiberiu Georgescu wrote: >>> When a page allocated using the MAP_SHARED flag is swapped out, its pagemap >>> entry is cleared. In many cases, there is no difference between swapped-out >>> shared pages and newly allocated, non-dirty pages in the pagemap interface. >>> >>> This patch addresses the behaviour and modifies pte_to_pagemap_entry() to >>> make use of the XArray associated with the virtual memory area struct >>> passed as an argument. The XArray contains the location of virtual pages >>> in the page cache, swap cache or on disk. If they are on either of the >>> caches, then the original implementation still works. If not, then the >>> missing information will be retrieved from the XArray. >>> >>> Co-developed-by: Florian Schmidt <florian.schmidt@nutanix.com> >>> Signed-off-by: Florian Schmidt <florian.schmidt@nutanix.com> >>> Co-developed-by: Carl Waldspurger <carl.waldspurger@nutanix.com> >>> Signed-off-by: Carl Waldspurger <carl.waldspurger@nutanix.com> >>> Co-developed-by: Ivan Teterevkov <ivan.teterevkov@nutanix.com> >>> Signed-off-by: Ivan Teterevkov <ivan.teterevkov@nutanix.com> >>> Signed-off-by: Tiberiu Georgescu <tiberiu.georgescu@nutanix.com> >>> --- >>> fs/proc/task_mmu.c | 37 +++++++++++++++++++++++++++++-------- >>> 1 file changed, 29 insertions(+), 8 deletions(-) >>> >>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c >>> index eb97468dfe4c..b17c8aedd32e 100644 >>> --- a/fs/proc/task_mmu.c >>> +++ b/fs/proc/task_mmu.c >>> @@ -1359,12 +1359,25 @@ static int pagemap_pte_hole(unsigned long start, unsigned long end, >>> return err; >>> } >>> >>> +static void *get_xa_entry_at_vma_addr(struct vm_area_struct *vma, >>> + unsigned long addr) >>> +{ >>> + struct inode *inode = file_inode(vma->vm_file); >>> + struct address_space *mapping = inode->i_mapping; >>> + pgoff_t offset = linear_page_index(vma, addr); >>> + >>> + return xa_load(&mapping->i_pages, offset); >>> +} >>> + >>> static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm, >>> struct vm_area_struct *vma, unsigned long addr, pte_t pte) >>> { >>> u64 frame = 0, flags = 0; >>> struct page *page = NULL; >>> >>> + if (vma->vm_flags & VM_SOFTDIRTY) >>> + flags |= PM_SOFT_DIRTY; >>> + >>> if (pte_present(pte)) { >>> if (pm->show_pfn) >>> frame = pte_pfn(pte); >>> @@ -1374,13 +1387,22 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm, >>> flags |= PM_SOFT_DIRTY; >>> if (pte_uffd_wp(pte)) >>> flags |= PM_UFFD_WP; >>> - } else if (is_swap_pte(pte)) { >>> + } else if (is_swap_pte(pte) || shmem_file(vma->vm_file)) { >>> swp_entry_t entry; >>> - if (pte_swp_soft_dirty(pte)) >>> - flags |= PM_SOFT_DIRTY; >>> - if (pte_swp_uffd_wp(pte)) >>> - flags |= PM_UFFD_WP; >>> - entry = pte_to_swp_entry(pte); >>> + if (is_swap_pte(pte)) { >>> + entry = pte_to_swp_entry(pte); >>> + if (pte_swp_soft_dirty(pte)) >>> + flags |= PM_SOFT_DIRTY; >>> + if (pte_swp_uffd_wp(pte)) >>> + flags |= PM_UFFD_WP; >>> + } else { >>> + void *xa_entry = get_xa_entry_at_vma_addr(vma, addr); >>> + >>> + if (xa_is_value(xa_entry)) >>> + entry = radix_to_swp_entry(xa_entry); >>> + else >>> + goto out; >>> + } >>> if (pm->show_pfn) >>> frame = swp_type(entry) | >>> (swp_offset(entry) << MAX_SWAPFILES_SHIFT); >>> @@ -1393,9 +1415,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm, >>> flags |= PM_FILE; >>> if (page && page_mapcount(page) == 1) >>> flags |= PM_MMAP_EXCLUSIVE; >>> - if (vma->vm_flags & VM_SOFTDIRTY) >>> - flags |= PM_SOFT_DIRTY; >> >> IMHO moving this to the entry will only work for the initial iteration, however >> it won't really help anything, as soft-dirty should always be used in pair with >> clear_refs written with value "4" first otherwise all pages will be marked >> soft-dirty then the pagemap data is meaningless. >> >> After the "write 4" op VM_SOFTDIRTY will be cleared and I expect the test case >> to see all zeros again even with the patch. >> >> I think one way to fix this is to do something similar to uffd-wp: we leave a >> marker in pte showing that this is soft-dirtied pte even if swapped out. > > How exactly does such a pte look like? Simply pte_none() with another > bit set? > >> However we don't have a mechanism for that yet in current linux, and the >> uffd-wp series is the first one trying to introduce something like that. > > Can you give me a pointer? I'm very interested in learning how to > identify this case. >
I assume it's https://lore.kernel.org/lkml/20210527202117.30689-1-peterx@redhat.com/
-- Thanks,
David / dhildenb
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