Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

["Zach O'Keefe" <zokeefe@google.com>]

On Mon, Aug 14, 2023 at 12:06 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Mon, Aug 14, 2023 at 11:47:50AM -0700, Zach O'Keefe wrote:
> > Willy -- I'm not up-to-date on what is happening on the THP-fs front.
> > Should we be checking for a ->huge_fault handler here?
>
> Oh, thank goodness, I thought you were cc'ing me to ask a DAX question ...

:)

> From a large folios perspective, filesystems do not implement a special
> handler.  They call filemap_fault() (directly or indirectly) from their
> ->fault handler.  If there is already a folio in the page cache which
> satisfies this fault, we insert it into the page tables (no matter what
> size it is).  If there is no folio, we call readahead to populate that
> index in the page cache, and probably some other indices around it.
> That's do_sync_mmap_readahead().
>
> If you look at that, you'll see that we check the VM_HUGEPAGE flag, and
> if set we align to a PMD boundary and read two PMD-size pages (so that we
> can do async readahead for the second page, if we're doing a linear scan).
> If the VM_HUGEPAGE flag isn't set, we'll use the readahead algorithm to
> decide how large the folio should be that we're reading into; if it's a
> random read workload, we'll stick to order-0 pages, but if we're getting
> good hit rate from the linear scan, we'll increase the size (although
> we won't go past PMD size)
>
> There's also the ->map_pages() optimisation which handles page faults
> locklessly, and will fail back to ->fault() if there's even a light
> breeze.  I don't think that's of any particular use in answering your
> question, so I'm not going into details about it.
>
> I'm not sure I understand the code that's being modified well enough to
> be able to give you a straight answer to your question, but hopefully
> this is helpful to you.

Thank you, this was great info. I had thought, incorrectly, that large
folio work would eventually tie into that ->huge_fault() handler
(should be dax_huge_fault() ?)

If that's the case, then faulting file-backed, non-DAX memory as
(pmd-mapped-)THPs isn't supported at all, and no fault lies with the
aforementioned patches.

Saurabh, perhaps you can elaborate on your use case a bit more, and
how that anonymous check broke you?

Best,
Zach