Re: [PATCH] mm: remove VM_EXEC requirement for THP eligibility

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 20, 2023 at 08:53:38PM -0800, Fangrui Song wrote:
> Thanks for the comment. Frankly, I am not familiar with huge pages...
> I noticed this VM_EXEC condition when I was writing this
> hugepage-related section in
> https://maskray.me/blog/2023-12-17-exploring-the-section-layout-in-linker-output#transparent-huge-pages-for-mapped-files
> (Thanks to Alexander Monakov's comment about
> CONFIG_READ_ONLY_THP_FOR_FS in
> https://mazzo.li/posts/check-huge-page.html).

CONFIG_READ_ONLY_THP_FOR_FS is a preliminary hack which solves some
problems.  The real solution is using large folios, which at the moment
means that you should test on XFS or AFS; filesystem authors have not
been enthusiastic about adding support to their filesystems so far.

In your blog, you write:

: In -z noseparate-code layouts, the file content starts somewhere at
: the first page, potentially wasting half a huge page on unrelated
: content. Switching to -z separate-code allows reclaiming the benefits
: of the half huge page but increases the file size. Balancing
: these aspects poses a challenge. One potential solution is using
: fallocate(FALLOC_FL_PUNCH_HOLE), which introduces complexity into the
: linker. However, this approach feels like a workaround to address a
: kernel limitation. It would be preferable if a file-backed huge page
: didn't necessitate a file offset aligned to a huge page boundary.

You should distinguish between file size (ie st_size in stat(3)) and
amount of space occupied on storage (st_blocks).  The linker should be
fine with creating a sparse file.  If it doesn't, cp --sparse will do
the trick.

Yes, it's a kernel limitation that folios have to be aligned within the
file as well as in both virtual and physical address space.  It's a huge
complexity win to do that; I don't think we'd be able to tile the page
cache effectively if we allowed folios to be placed at arbitrary offsets
(I think it turns into a knapsack problem at that point).

> As dTLB for read-only data is also an important optimization of
> file-backed THP, it seems straightforward that we should drop the
> VM_EXEC condition :)

I'm not particularly enthusiastic about making CONFIG_READ_ONLY_THP_FOR_FS
better.  Large folios are the future.  Indeed, I'd like to see
CONFIG_READ_ONLY_THP_FOR_FS go away in the next year or two once
btrfs and ext4 have support for large folios.