Re: [PATCH v4 0/6] add mTHP support for anonymous shmem

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2024/6/10 20:10, Daniel Gomez wrote:
> Hi Baolin,
> 
> On Tue, Jun 04, 2024 at 06:17:44PM +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.
>>
>> 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.
>>
>> 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.
>>
>> Use the page fault latency tool to measure the performance of 1G anonymous shmem
> 
> I'm not familiar with this tool. Could you share which repo/tool you are
> referring to?

Sure. The git repo is: https://github.com/gormanm/pft.git

And I did a little changes to test anon shmem:
diff --git a/pft.c b/pft.c
index 3ab1457..bbcd7e6 100644
--- a/pft.c
+++ b/pft.c
@@ -739,7 +739,7 @@ alloc_test_memory(void)
         int j;

         if (do_shm) {
-               if (p = alloc_shm(bytes)) {
+               if (p = valloc_shared(bytes)) {
                         do_mbind(p, bytes);
                         do_noclear(p, bytes);
                 }

> Also, are you running or are you aware of any other tools/tests available for
> shmem that we can use to make sure we do not introduce any regressions?

I did run the mm selftest cases, as well as some testing cases I wrote 
for anon shmem.