[PATCH v1] mm: shmem: Rename mTHP shmem counters
From: Barry Song
Date: Mon Jul 08 2024 - 21:44:54 EST
On Tue, Jul 9, 2024 at 12:30 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 08/07/2024 12:36, Barry Song wrote:
> > On Mon, Jul 8, 2024 at 11:24 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >>
> >> The legacy PMD-sized THP counters at /proc/vmstat include
> >> thp_file_alloc, thp_file_fallback and thp_file_fallback_charge, which
> >> rather confusingly refer to shmem THP and do not include any other types
> >> of file pages. This is inconsistent since in most other places in the
> >> kernel, THP counters are explicitly separated for anon, shmem and file
> >> flavours. However, we are stuck with it since it constitutes a user ABI.
> >>
> >> Recently, commit 66f44583f9b6 ("mm: shmem: add mTHP counters for
> >> anonymous shmem") added equivalent mTHP stats for shmem, keeping the
> >> same "file_" prefix in the names. But in future, we may want to add
> >> extra stats to cover actual file pages, at which point, it would all
> >> become very confusing.
> >>
> >> So let's take the opportunity to rename these new counters "shmem_"
> >> before the change makes it upstream and the ABI becomes immutable.
> >
> > Personally, I think this approach is much clearer. However, I recall
> > we discussed this
> > before [1], and it seems that inconsistency is a concern?
>
> Embarrassingly, I don't recall that converstation at all :-| but at least what I
> said then is consistent with what I've done in this patch.
>
> I think David's conclusion from that thread was to call them FILE_, and add both
> shmem and pagecache counts to those counters, meaning we can keep the same name
> as legacy THP counters. But those legacy THP counters only count shmem, and I
> don't think we would get away with adding pagecache counts to those at this
> point? (argument: they have been around for long time and there is a risk that
> user space relies on them and if they were to dramatically increase due to
> pagecache addition now that could break things). In that case, there is still
> inconsistency, but its worse; the names are consistent but the semantics are
> inconsistent.
>
> So my vote is to change to SHMEM_ as per this patch :)
I have no objections. However, I dislike the documentation for
thp_file_*. Perhaps we can clean it all up together ?
diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
index 709fe10b60f4..65df48cb3bbb 100644
--- a/Documentation/admin-guide/mm/transhuge.rst
+++ b/Documentation/admin-guide/mm/transhuge.rst
@@ -417,21 +417,22 @@ thp_collapse_alloc_failed
the allocation.
thp_file_alloc
- is incremented every time a file huge page is successfully
- allocated.
+ is incremented every time a file (including shmem) huge page is
+ successfully allocated.
thp_file_fallback
- is incremented if a file huge page is attempted to be allocated
- but fails and instead falls back to using small pages.
+ is incremented if a file (including shmem) huge page is attempted
+ to be allocated but fails and instead falls back to using small
+ pages.
thp_file_fallback_charge
- is incremented if a file huge page cannot be charged and instead
- falls back to using small pages even though the allocation was
- successful.
+ is incremented if a file (including shmem) huge page cannot be
+ charged and instead falls back to using small pages even though
+ the allocation was successful.
thp_file_mapped
- is incremented every time a file huge page is mapped into
- user address space.
+ is incremented every time a file (including shmem) huge page is
+ mapped into user address space.
thp_split_page
is incremented every time a huge page is split into base
>
> >
> > [1] https://lore.kernel.org/linux-mm/05d0096e4ec3e572d1d52d33a31a661321ac1551.1713755580.git.baolin.wang@xxxxxxxxxxxxxxxxx/
> >
> >
> >>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >> ---
> >>
> >> Hi All,
> >>
> >> Applies on top of today's mm-unstable (2073cda629a4) and tested with mm
> >> selftests; no regressions observed.
> >>
> >> The backstory here is that I'd like to introduce some counters for regular file
> >> folio allocations to observe how often large folio allocation succeeds, but
> >> these shmem counters are named "file" which is going to make things confusing.
> >> So hoping to solve that before commit 66f44583f9b6 ("mm: shmem: add mTHP
> >> counters for anonymous shmem") goes upstream (it is currently in mm-stable).
> >>
> >> Admittedly, this change means the mTHP stat names are not the same as the legacy
> >> PMD-size THP names, but I think that's a smaller issue than having "file_" mTHP
> >> stats that only count shmem, then having to introduce "file2_" or "pgcache_"
> >> stats for the regular file memory, which is even more inconsistent IMHO. I guess
> >> the alternative is to count both shmem and file in these mTHP stats (that's how
> >> they were documented anyway) but I think it's better to be able to consider them
> >> separately like we do for all the other counters.
> >>
> >> Thanks,
> >> Ryan
> >>
> >> Documentation/admin-guide/mm/transhuge.rst | 12 ++++++------
> >> include/linux/huge_mm.h | 6 +++---
> >> mm/huge_memory.c | 12 ++++++------
> >> mm/shmem.c | 8 ++++----
> >> 4 files changed, 19 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst
> >> index 747c811ee8f1..8b891689fc13 100644
> >> --- a/Documentation/admin-guide/mm/transhuge.rst
> >> +++ b/Documentation/admin-guide/mm/transhuge.rst
> >> @@ -496,16 +496,16 @@ swpout_fallback
> >> Usually because failed to allocate some continuous swap space
> >> for the huge page.
> >>
> >> -file_alloc
> >> - is incremented every time a file huge page is successfully
> >> +shmem_alloc
> >> + is incremented every time a shmem huge page is successfully
> >> allocated.
> >>
> >> -file_fallback
> >> - is incremented if a file huge page is attempted to be allocated
> >> +shmem_fallback
> >> + is incremented if a shmem huge page is attempted to be allocated
> >> but fails and instead falls back to using small pages.
> >>
> >> -file_fallback_charge
> >> - is incremented if a file huge page cannot be charged and instead
> >> +shmem_fallback_charge
> >> + is incremented if a shmem huge page cannot be charged and instead
> >> falls back to using small pages even though the allocation was
> >> successful.
> >>
> >> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> >> index acb6ac24a07e..cff002be83eb 100644
> >> --- a/include/linux/huge_mm.h
> >> +++ b/include/linux/huge_mm.h
> >> @@ -269,9 +269,9 @@ enum mthp_stat_item {
> >> MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE,
> >> MTHP_STAT_SWPOUT,
> >> MTHP_STAT_SWPOUT_FALLBACK,
> >> - MTHP_STAT_FILE_ALLOC,
> >> - MTHP_STAT_FILE_FALLBACK,
> >> - MTHP_STAT_FILE_FALLBACK_CHARGE,
> >> + MTHP_STAT_SHMEM_ALLOC,
> >> + MTHP_STAT_SHMEM_FALLBACK,
> >> + MTHP_STAT_SHMEM_FALLBACK_CHARGE,
> >> MTHP_STAT_SPLIT,
> >> MTHP_STAT_SPLIT_FAILED,
> >> MTHP_STAT_SPLIT_DEFERRED,
> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> >> index 9ec64aa2be94..f9696c94e211 100644
> >> --- a/mm/huge_memory.c
> >> +++ b/mm/huge_memory.c
> >> @@ -568,9 +568,9 @@ DEFINE_MTHP_STAT_ATTR(anon_fault_fallback, MTHP_STAT_ANON_FAULT_FALLBACK);
> >> DEFINE_MTHP_STAT_ATTR(anon_fault_fallback_charge, MTHP_STAT_ANON_FAULT_FALLBACK_CHARGE);
> >> DEFINE_MTHP_STAT_ATTR(swpout, MTHP_STAT_SWPOUT);
> >> DEFINE_MTHP_STAT_ATTR(swpout_fallback, MTHP_STAT_SWPOUT_FALLBACK);
> >> -DEFINE_MTHP_STAT_ATTR(file_alloc, MTHP_STAT_FILE_ALLOC);
> >> -DEFINE_MTHP_STAT_ATTR(file_fallback, MTHP_STAT_FILE_FALLBACK);
> >> -DEFINE_MTHP_STAT_ATTR(file_fallback_charge, MTHP_STAT_FILE_FALLBACK_CHARGE);
> >> +DEFINE_MTHP_STAT_ATTR(shmem_alloc, MTHP_STAT_SHMEM_ALLOC);
> >> +DEFINE_MTHP_STAT_ATTR(shmem_fallback, MTHP_STAT_SHMEM_FALLBACK);
> >> +DEFINE_MTHP_STAT_ATTR(shmem_fallback_charge, MTHP_STAT_SHMEM_FALLBACK_CHARGE);
> >> DEFINE_MTHP_STAT_ATTR(split, MTHP_STAT_SPLIT);
> >> DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED);
> >> DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED);
> >> @@ -581,9 +581,9 @@ static struct attribute *stats_attrs[] = {
> >> &anon_fault_fallback_charge_attr.attr,
> >> &swpout_attr.attr,
> >> &swpout_fallback_attr.attr,
> >> - &file_alloc_attr.attr,
> >> - &file_fallback_attr.attr,
> >> - &file_fallback_charge_attr.attr,
> >> + &shmem_alloc_attr.attr,
> >> + &shmem_fallback_attr.attr,
> >> + &shmem_fallback_charge_attr.attr,
> >> &split_attr.attr,
> >> &split_failed_attr.attr,
> >> &split_deferred_attr.attr,
> >> diff --git a/mm/shmem.c b/mm/shmem.c
> >> index 921d59c3d669..f24dfbd387ba 100644
> >> --- a/mm/shmem.c
> >> +++ b/mm/shmem.c
> >> @@ -1777,7 +1777,7 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
> >> if (pages == HPAGE_PMD_NR)
> >> count_vm_event(THP_FILE_FALLBACK);
> >> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >> - count_mthp_stat(order, MTHP_STAT_FILE_FALLBACK);
> >> + count_mthp_stat(order, MTHP_STAT_SHMEM_FALLBACK);
> >> #endif
> >> order = next_order(&suitable_orders, order);
> >> }
> >> @@ -1804,8 +1804,8 @@ static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf,
> >> count_vm_event(THP_FILE_FALLBACK_CHARGE);
> >> }
> >> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >> - count_mthp_stat(folio_order(folio), MTHP_STAT_FILE_FALLBACK);
> >> - count_mthp_stat(folio_order(folio), MTHP_STAT_FILE_FALLBACK_CHARGE);
> >> + count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK);
> >> + count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_FALLBACK_CHARGE);
> >> #endif
> >> }
> >> goto unlock;
> >> @@ -2181,7 +2181,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
> >> if (folio_test_pmd_mappable(folio))
> >> count_vm_event(THP_FILE_ALLOC);
> >> #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> >> - count_mthp_stat(folio_order(folio), MTHP_STAT_FILE_ALLOC);
> >> + count_mthp_stat(folio_order(folio), MTHP_STAT_SHMEM_ALLOC);
> >> #endif
> >> goto alloced;
> >> }
> >> --
> >> 2.43.0
> >>
> >
Thanks
Barry