Re: [PATCH v5 0/6] add mTHP support for anonymous shmem

[Matthew Wilcox <willy@infradead.org>]

On Tue, Jun 11, 2024 at 06:11:04PM +0800, Baolin Wang wrote:
> Anonymous pages have already been supported for multi-size (mTHP) allocation
> through commit 19eaf44954df, that can allow THP to be configured through the
> sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.
> 
> However, the anonymous shmem will ignore the anonymous mTHP rule configured
> through the sysfs interface, and can only use the PMD-mapped THP, that is not
> reasonable. Many implement anonymous page sharing through mmap(MAP_SHARED |
> MAP_ANONYMOUS), especially in database usage scenarios, therefore, users expect
> to apply an unified mTHP strategy for anonymous pages, also including the
> anonymous shared pages, in order to enjoy the benefits of mTHP. For example,
> lower latency than PMD-mapped THP, smaller memory bloat than PMD-mapped THP,
> contiguous PTEs on ARM architecture to reduce TLB miss etc.

OK, this makes sense.

> As discussed in the bi-weekly MM meeting[1], the mTHP controls should control
> all of shmem, not only anonymous shmem, but support will be added iteratively.
> Therefore, this patch set starts with support for anonymous shmem.

But then this doesn't.  You say first that users want the same controls
to control all anonymous memory, then you introduce a completely
separate set of controls for shared anonymous memory.

shmem has two uses:

 - MAP_ANONYMOUS | MAP_SHARED (this patch set)
 - tmpfs

For the second use case we don't want controls *at all*, we want the
same heiristics used for all other filesystems to apply to tmpfs.

There's no reason to have separate controls for choosing folio size
in shmem.

> The primary strategy is similar to supporting anonymous mTHP. Introduce
> a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled',
> which can have almost the same values as the top-level
> '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new
> additional "inherit" option and dropping the testing options 'force' and
> 'deny'. By default all sizes will be set to "never" except PMD size, which
> is set to "inherit". This ensures backward compatibility with the anonymous
> shmem enabled of the top level, meanwhile also allows independent control of
> anonymous shmem enabled for each mTHP.