Re: [PATCH RFC v2 6/9] mm/khugepaged: remove reuse_swap_page() usage

[Yang Shi]

On Fri, Jan 28, 2022 at 12:41 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 27.01.22 22:23, Yang Shi wrote:
> > On Wed, Jan 26, 2022 at 2:00 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> reuse_swap_page() currently indicates if we can write to an anon page
> >> without COW. A COW is required if the page is shared by multiple
> >> processes (either already mapped or via swap entries) or if there is
> >> concurrent writeback that cannot tolerate concurrent page modifications.
> >>
> >> reuse_swap_page() doesn't check for pending references from other
> >> processes that already unmapped the page, however,
> >> is_refcount_suitable() essentially does the same thing in the context of
> >> khugepaged. khugepaged is the last remaining user of reuse_swap_page() and
> >> we want to remove that function.
> >>
> >> In the context of khugepaged, we are not actually going to write to the
> >> page and we don't really care about other processes mapping the page:
> >> for example, without swap, we don't care about shared pages at all.
> >>
> >> The current logic seems to be:
> >> * Writable: -> Not shared, but might be in the swapcache. Nobody can
> >>   fault it in from the swapcache as there are no other swap entries.
> >> * Readable and not in the swapcache: Might be shared (but nobody can
> >>   fault it in from the swapcache).
> >> * Readable and in the swapcache: Might be shared and someone might be
> >>   able to fault it in from the swapcache. Make sure we're the exclusive
> >>   owner via reuse_swap_page().
> >>
> >> Having to guess due to lack of comments and documentation, the current
> >> logic really only wants to make sure that a page that might be shared
> >> cannot be faulted in from the swapcache while khugepaged is active.
> >> It's hard to guess why that is that case and if it's really still required,
> >> but let's try keeping that logic unmodified.
> >
> > I don't think it could be faulted in while khugepaged is active since
> > khugepaged does hold mmap_lock in write mode IIUC. So page fault is
> > serialized against khugepaged.
>
> It could get faulted in by another process sharing the page, because we
> only synchronize against the current process.

Yes, sure. I'm supposed you meant do_swap_page() called by another
process. But it is serialized by page lock. So khugepaged won't see
something in limbo state IIUC.

>
> >
> > My wild guess is that collapsing shared pages was not supported before
> > v5.8, so we need reuse_swap_page() to tell us if the page in swap
> > cache is shared or not. But it is not true anymore. And khugepaged
> > just allocates a THP then copy the data from base pages to huge page
> > then replace PTEs to PMD, it doesn't change the content of the page,
> > so I failed to see a problem by collapsing a shared page in swap
> > cache. But I'm really not entirely sure, I may miss something...
>
>
> Looking more closely where this logic originates from, it was introduced in:
>
> commit 10359213d05acf804558bda7cc9b8422a828d1cd
> Author: Ebru Akagunduz <ebru.akagunduz@xxxxxxxxx>
> Date:   Wed Feb 11 15:28:28 2015 -0800
>
>     mm: incorporate read-only pages into transparent huge pages
>
>     This patch aims to improve THP collapse rates, by allowing THP collapse in
>     the presence of read-only ptes, like those left in place by do_swap_page
>     after a read fault.
>
>     Currently THP can collapse 4kB pages into a THP when there are up to
>     khugepaged_max_ptes_none pte_none ptes in a 2MB range.  This patch applies
>     the same limit for read-only ptes.
>
>
> The change essentially results in a read-only mapped PTE page getting copied and
> mapped writable via a new PMD-mapped THP.
>
> It mentions do_swap_page(), so I assume it just tried to do what do_swap_page()
> would do when trying to map a page swapped in from the page cache writable
> immediately.
>
> But we differ from do_swap_page() that we're not actually going to map the page
> writable, we're going to copy the page (__collapse_huge_page_copy()) and map
> the copy writable.

Yeah, this is the point. Khugepaged or the process being collapsed
won't write to the original page. Just unshare it.

>
> I assume we can remove that logic.
>
> --
> Thanks,
>
> David / dhildenb
>