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SubjectRe: [RFC PATCH 1/1] pagemap: report swap location for shared pages
Date
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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