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Message-ID: <YO8a4FpvBVEIBgjK@t490s>
Date: Wed, 14 Jul 2021 13:12:00 -0400
From: Peter Xu <peterx@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: Tiberiu Georgescu <tiberiu.georgescu@...anix.com>,
akpm@...ux-foundation.org, catalin.marinas@....com,
peterz@...radead.org, chinwen.chang@...iatek.com,
linmiaohe@...wei.com, jannh@...gle.com, apopple@...dia.com,
christian.brauner@...ntu.com, ebiederm@...ssion.com,
adobriyan@...il.com, songmuchun@...edance.com, axboe@...nel.dk,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, ivan.teterevkov@...anix.com,
florian.schmidt@...anix.com, carl.waldspurger@...anix.com,
Hugh Dickins <hughd@...gle.com>,
Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [RFC PATCH 1/1] pagemap: report swap location for shared pages
On Wed, Jul 14, 2021 at 06:30:05PM +0200, David Hildenbrand wrote:
> 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@...anix.com>
> > > > Signed-off-by: Florian Schmidt <florian.schmidt@...anix.com>
> > > > Co-developed-by: Carl Waldspurger <carl.waldspurger@...anix.com>
> > > > Signed-off-by: Carl Waldspurger <carl.waldspurger@...anix.com>
> > > > Co-developed-by: Ivan Teterevkov <ivan.teterevkov@...anix.com>
> > > > Signed-off-by: Ivan Teterevkov <ivan.teterevkov@...anix.com>
> > > > Signed-off-by: Tiberiu Georgescu <tiberiu.georgescu@...anix.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?
Yes something like that. The pte can be defined at will, as long as never used
elsewhere.
> >
> > > 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/
Yes.
--
Peter Xu
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