Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [RFC PATCH 1/1] pagemap: report swap location for shared pages | Date | Wed, 14 Jul 2021 18:24:02 +0200 |
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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.
-- Thanks,
David / dhildenb
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