Re: [PATCH v2] mm/swap: fix race when skipping swapcache

[Minchan Kim]

Hi David,

On Thu, Feb 15, 2024 at 09:03:28PM +0100, David Hildenbrand wrote:

< snip >

> > > 
> > > We would detect later, that the PTE changed, but we would temporarily
> > > mess with that swap slot that we might no longer "own".
> > > 
> > > I was thinking about alternatives, it's tricky because of the concurrent
> > > MADV_DONTNEED possibility. Something with another fake-swap entry type
> > > (similar to migration entries) might work, but would require more changes.
> > 
> > Yeah, in the long term I also think more work is needed for the swap subsystem.
> > 
> > In my opinion, for this particular issue, or, for cache bypassed
> > swapin, a new swap map value similar to SWAP_MAP_BAD/SWAP_MAP_SHMEM
> > might be needed, that may even help to simplify the swap count release
> > routine for cache bypassed swapin, and improve the performance.
> 
> The question is if we really want to track that in the swapcache and not
> rather in the page table.
> 
> Imagine the following:
> 
> (1) allocate the folio and lock it (we do that already)
> 
> (2) take the page table lock. If the PTE is still the same, insert a new
> "swapin_in_process" fake swp entry that references the locked folio.
> 
> (3) read the folio from swap. This will unlock the folio IIUC. (we do that
> already)
> 
> (4) relock the folio. (we do that already, might not want to fail)
> 
> (4) take the PTE lock. If the PTE did not change, turn it into a present PTE
> entry. Otherwise, cleanup.
> 
> 
> Any concurrent swap-in users would spot the new "swapin_in_process" fake swp
> entry and wait for the page lock (just like we do with migration entries).
> 
> Zap code would mostly only clear the "swapin_in_process" fake swp entry and
> leave the cleanup to (4) above. Fortunately, concurrent fork() is impossible
> as that cannot race with page faults.
> 
> There might be one minor thing to optimize with the folio lock above. But in
> essence, it would work just like migration entries, just that they are
> installed only while we actually do read the content from disk etc.

That's a great idea. I was thinking to have the synchronization in the
page table but couldn't reach to the other non_swap_entry idea.

Only concern of the approach is that it would be harder to have the fix
in the stable tree. If there isn't strong objection, I prefer the
Kairui's orginal solution(with some tweak of scheduler if it's
necessary) first and then pursue your idea on latest tree.