On 2024/4/24 16:26, Ryan Roberts wrote:
On 24/04/2024 07:55, Baolin Wang wrote:
On 2024/4/23 18:41, Ryan Roberts wrote:
On 22/04/2024 08:02, 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 shared pages 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.
This sounds like a very useful addition!
Out of interest, can you point me at any workloads (and off-the-shelf benchmarks
for those workloads) that predominantly use shared anon memory?
As far as I know, some database related workloads make extensive use of shared
anonymous page, such as PolarDB[1] in our Alibaba fleet, or MySQL likely also
uses shared anonymous memory. And I still need to do some investigation to
measure the performance.
[1] https://github.com/ApsaraDB/PolarDB-for-PostgreSQL
Thanks for the pointer!
The primary strategy is that, the use of huge pages for anonymous shared pages
still follows the global control determined by the mount option "huge="
parameter
or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
The utilization of mTHP is allowed only when the global 'huge' switch is
enabled.
Subsequently, the mTHP sysfs interface
(/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled)
is checked to determine the mTHP size that can be used for large folio
allocation
for these anonymous shared pages.
I'm not sure about this proposed control mechanism; won't it break
compatibility? I could be wrong, but I don't think shmem's use of THP used to
depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it
Yes, I realized this after more testing.
doesn't make sense to me that we now depend upon the
/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by
default disables all sizes except 2M, which is set to "inherit" from
/sys/kernel/mm/transparent_hugepage/enabled).
The other problem is that shmem_enabled has a different set of options
(always/never/within_size/advise/deny/force) to enabled (always/madvise/never)
Perhaps it would be cleaner to do the same trick we did for enabled; Introduce
/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the
same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled,
plus the additional "inherit" option. By default all sizes will be set to
"never" except 2M, which is set to "inherit".
Sounds good to me. But I do not want to copy all same values from top-level
'/sys/kernel/mm/transparent_hugepage/shmem_enabled':
always within_size advise never deny force
For mTHP's shmem_enabled interface, we can just keep below values:
always within_size advise never
Cause when checking if mTHP can be used for anon shmem, 'deny' is equal to
'never', and 'force' is equal to 'always'.
I'll admit it wasn't completely clear to me after reading the docs, but my rough
understanding is:
- /sys/kernel/mm/transparent_hugepage/shmem_enabled controls
mmap(SHARED|ANON) allocations (mostly; see rule 3)
- huge=... controls tmpfs allocations
- deny and force in shmem_enabled are equivalent to never and always for
mmap(SHARED|ANON) but additionally override all tmpfs mounts so they act as
if they were mounted with huge=never or huge=always
Is that correct? If so, then I think it still makes sense to support per-size
Correct.
deny/force. Certainly if a per-size control is set to "inherit" and the
top-level control is set to deny or force, you would need that to mean something.
IMHO, the '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled'
interface should only control the anonymous shmem. And 'huge=' controls
tmpfs allocation, so we should not use anonymous control to override
tmpfs control, which seems a little mess?
Of course the huge= mount option would also need to take a per-size option in
this case. e.g. huge=2048kB:advise,64kB:always
IMO, I do not want to change the global 'huge=' mount option, which can control
both anon shmem and tmpfs, but mTHP now is only applied for anon shmem. So let's
How does huge= control anon shmem? I thought it was only for mounted
filesystems; so tmpfs? Perhaps my mental model for how this works is broken...
Sorry for noise, you are right. So this is still the reason I don't want
to change the semantics of 'huge=', which is used to control tmpfs.
keep it be same with the global sysfs interface:
/sys/kernel/mm/transparent_hugepage/shmem_enabled.
For tmpfs large folio strategy, I plan to address it later, and we may need more
discussion to determine if it should follow the file large folio strategy or not
(no investigation now).
OK. But until you get to tmpfs, you'll need an interim definition for what it
means if a per-size control is set to "inherit" and the top-level control is set
to deny/force.
Thanks for reviewing.
No problem! Thanks for doing the work!
TODO:
- More testing and provide some performance data.
- Need more discussion about the large folio allocation strategy for a
'regular
file' operation created by memfd_create(), for example using ftruncate(fd) to
specify
the 'file' size, which need to follow the anonymous mTHP rule too?
- Do not split the large folio when share memory swap out.
- Can swap in a large folio for share memory.
Baolin Wang (5):
mm: memory: extend finish_fault() to support large folio
mm: shmem: add an 'order' parameter for shmem_alloc_hugefolio()
mm: shmem: add THP validation for PMD-mapped THP related statistics
mm: shmem: add mTHP support for anonymous share pages
mm: shmem: add anonymous share mTHP counters
include/linux/huge_mm.h | 4 +-
mm/huge_memory.c | 8 ++-
mm/memory.c | 25 +++++++---
mm/shmem.c | 107 ++++++++++++++++++++++++++++++----------
4 files changed, 108 insertions(+), 36 deletions(-)
