Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures, honor failures

[Pavel Tatashin]

On Wed, Jan 13, 2021 at 3:05 PM Pavel Tatashin
<pasha.tatashin@xxxxxxxxxx> wrote:
>
> > > > Oh, that existing logic is wrong too :( Another bug.
> > >
> > > I do not think there is a bug.
> > >
> > > > You can't skip pages in the pages[] array under the assumption they
> > > > are contiguous. ie the i+=step is wrong.
> > >
> > > If pages[i] is part of a compound page, the other parts of this page
> > > must be sequential in this array for this compound page
> >
> > That is true only if the PMD points to the page. If the PTE points to
> > a tail page then there is no requirement that other PTEs are
> > contiguous with the compount page.
> >
> > At this point we have no idea if the GUP logic got this compound page
> > as a head page in a PMD or as a tail page from a PTE, so we can't
> > assume a contiguous run of addresses.
>
> I see, I will fix this bug in an upstream as a separate patch in my
> series, and keep the fix when my fixes are applied.
>
> >
> > Look at split_huge_pmd() - it doesn't break up the compound page it
> > just converts the PMD to a PTE array and scatters the tail pages to
> > the PTE.

Hi Jason,

I've been thinking about this some more. Again, I am not sure this is
a bug. I understand split_huge_pmd() may split the PMD size page into
PTEs and leave the compound page intact. However, in order for pages[]
to have non sequential addresses in compound page, those PTEs must
also be migrated after split_huge_pmd(), however when we migrate them
we will either migrate the whole compound page or do
split_huge_page_to_list() which will in turn do ClearPageCompound().
Please let me know if I am missing something.

Thank you,
Pasha

>
> Got it, unfortunately the fix will deoptimize the code by having to
> check every page if it is part of a previous compound page or not.
>
> >
> > I understand Matt is pushing on this idea more by having compound
> > pages in the page cache, but still mapping tail pages when required.
> >
> > > This is actually standard migration procedure, elsewhere in the kernel
> > > we migrate pages in exactly the same fashion: isolate and later
> > > migrate. The isolation works for LRU only pages.
> >
> > But do other places cause a userspace visible random failure when LRU
> > isolation fails?
>
> Makes sense, I will remove maximum retries for isolation, and retry
> indefinitely, the same as it is done during memory hot-remove. So, we
> will fail only when migration fails.
>
> >
> > I don't like it at all, what is the user supposed to do?
> >
> > Jason