linux-kernel - Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page

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Message-ID: <YwgFRLn43+U/hxwt@xz-m1.local>
Date:   Thu, 25 Aug 2022 19:27:00 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Nadav Amit <nadav.amit@...il.com>,
        huang ying <huang.ying.caritas@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        David Hildenbrand <david@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org, stable@...r.kernel.org,
        Huang Ying <ying.huang@...el.com>
Subject: Re: [PATCH v3 2/3] mm/migrate_device.c: Copy pte dirty bit to page

On Fri, Aug 26, 2022 at 08:21:44AM +1000, Alistair Popple wrote:
> 
> Peter Xu <peterx@...hat.com> writes:
> 
> > On Wed, Aug 24, 2022 at 01:03:38PM +1000, Alistair Popple wrote:
> >> migrate_vma_setup() has a fast path in migrate_vma_collect_pmd() that
> >> installs migration entries directly if it can lock the migrating page.
> >> When removing a dirty pte the dirty bit is supposed to be carried over
> >> to the underlying page to prevent it being lost.
> >>
> >> Currently migrate_vma_*() can only be used for private anonymous
> >> mappings. That means loss of the dirty bit usually doesn't result in
> >> data loss because these pages are typically not file-backed. However
> >> pages may be backed by swap storage which can result in data loss if an
> >> attempt is made to migrate a dirty page that doesn't yet have the
> >> PageDirty flag set.
> >>
> >> In this case migration will fail due to unexpected references but the
> >> dirty pte bit will be lost. If the page is subsequently reclaimed data
> >> won't be written back to swap storage as it is considered uptodate,
> >> resulting in data loss if the page is subsequently accessed.
> >>
> >> Prevent this by copying the dirty bit to the page when removing the pte
> >> to match what try_to_migrate_one() does.
> >>
> >> Signed-off-by: Alistair Popple <apopple@...dia.com>
> >> Acked-by: Peter Xu <peterx@...hat.com>
> >> Reported-by: Huang Ying <ying.huang@...el.com>
> >> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
> >> Cc: stable@...r.kernel.org
> >>
> >> ---
> >>
> >> Changes for v3:
> >>
> >>  - Defer TLB flushing
> >>  - Split a TLB flushing fix into a separate change.
> >>
> >> Changes for v2:
> >>
> >>  - Fixed up Reported-by tag.
> >>  - Added Peter's Acked-by.
> >>  - Atomically read and clear the pte to prevent the dirty bit getting
> >>    set after reading it.
> >>  - Added fixes tag
> >> ---
> >>  mm/migrate_device.c |  9 +++++++--
> >>  1 file changed, 7 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
> >> index 6a5ef9f..51d9afa 100644
> >> --- a/mm/migrate_device.c
> >> +++ b/mm/migrate_device.c
> >> @@ -7,6 +7,7 @@
> >>  #include <linux/export.h>
> >>  #include <linux/memremap.h>
> >>  #include <linux/migrate.h>
> >> +#include <linux/mm.h>
> >>  #include <linux/mm_inline.h>
> >>  #include <linux/mmu_notifier.h>
> >>  #include <linux/oom.h>
> >> @@ -196,7 +197,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >>  			anon_exclusive = PageAnon(page) && PageAnonExclusive(page);
> >>  			if (anon_exclusive) {
> >>  				flush_cache_page(vma, addr, pte_pfn(*ptep));
> >> -				ptep_clear_flush(vma, addr, ptep);
> >> +				pte = ptep_clear_flush(vma, addr, ptep);
> >>
> >>  				if (page_try_share_anon_rmap(page)) {
> >>  					set_pte_at(mm, addr, ptep, pte);
> >> @@ -206,11 +207,15 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
> >>  					goto next;
> >>  				}
> >>  			} else {
> >> -				ptep_get_and_clear(mm, addr, ptep);
> >> +				pte = ptep_get_and_clear(mm, addr, ptep);
> >>  			}
> >
> > I remember that in v2 both flush_cache_page() and ptep_get_and_clear() are
> > moved above the condition check so they're called unconditionally.  Could
> > you explain the rational on why it's changed back (since I think v2 was the
> > correct approach)?
> 
> Mainly because I agree with your original comments, that it would be
> better to keep the batching of TLB flushing if possible. After the
> discussion I don't think there is any issues with HW pte dirty bits
> here. There are already other cases where HW needs to get that right
> anyway (eg. zap_pte_range).

Yes tlb batching was kept, thanks for doing that way.  Though if only apply
patch 1 we'll have both ptep_clear_flush() and batched flush which seems to
be redundant.

> 
> > The other question is if we want to split the patch, would it be better to
> > move the tlb changes to patch 1, and leave the dirty bit fix in patch 2?
> 
> Isn't that already the case? Patch 1 moves the TLB flush before the PTL
> as suggested, patch 2 atomically copies the dirty bit without changing
> any TLB flushing.

IMHO it's cleaner to have patch 1 fix batch flush, replace
ptep_clear_flush() with ptep_get_and_clear() and update pte properly.

No strong opinions on the layout, but I still think we should drop the
redundant ptep_clear_flush() above, meanwhile add the flush_cache_page()
properly for !exclusive case too.

Thanks,

-- 
Peter Xu