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

[Pavel Tatashin]

> > > 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.

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