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b=rNufEJHE0To++g4VWWqZ+8sj2QU4KogfXr3w4jfj1OdfyL0UrxVoW+mbgM21ck+Cy kcIHXm6KwL4/JKtcEHWeUZT5u7tC4yWaVRgYaM1V4BT3sWV9HufJvcoV3pFKMk0MuQ vSWjwFgk+maDWtIICPeGVxnY0+x7Df2RY9obG+gNqGTSKfu2JmfZqe5VEbPItDp0kJ tVgbRYElPoIgkQ9y2Jw2RrKT13TTr24n7gj9J9w5vDsJ7zVFVTssBbLnCNNmtPjNRD M7k8hAgZbuQTmqaBk/ufG45TcPlngLq+vSASajVwMHNFXdcrbnzRGh3S4eU8mnl5Rk Fsd++iBZlBTDw== Date: Thu, 16 Apr 2026 08:21:07 +0100 From: Lorenzo Stoakes To: Nico Pache Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, david@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com Subject: Re: [PATCH mm-unstable v15 07/13] mm/khugepaged: add per-order mTHP collapse failure statistics Message-ID: References: <20260226031741.230674-1-npache@redhat.com> <20260226032504.233594-1-npache@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: A171D100007 X-Stat-Signature: d8diwti97xfweh9g9gdmxrj9aw14ckp4 X-Rspam-User: X-HE-Tag: 1776324083-443255 X-HE-Meta: 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 Zig8EjuH 2a26WJe4uDB55G8yu9wVsRIWIpyFesN7010Oo5ed4PNa0OQsr1hEFSb4b/xnu0iw67Be/8W0SokDTr8GvVAdyOOFehn56QhEJk69tHjdzshRjst2r5ZUbVc5ltwuyPgckQuRaQFla/dZy5ZSy1ULmddirt7fvCI4Z/XUFWsn7BTHsZ3B/a8uAR22Umrge32OLBxhtWM8sZYO1bs6WsvcuoPEXN5fo9hmUG3t4tBajvbaa5o5w9AnjPKRvFMM2xkJk7/nIrsjDoBWuu+Ej40m+28lNhKOQmmJhQ/xv2W7wedxuPiA6o3HQZ67ZtrBE6xknLdUAs1wn5NB/UFauADyzwcE/iBDGzTvlW2GnKczMxIRH1Cv2E56J1X6Kggtfvn7tBKfPQSvIpt6jDMmVf3mejbz8ybeXXAyM+M1d Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Ack on all below due to lower bandwidth :P It's nothing really major here so don't let any of this block on respin! Cheers, Lorenzo On Sun, Apr 12, 2026 at 08:48:29PM -0600, Nico Pache wrote: > On Tue, Mar 17, 2026 at 11:05 AM Lorenzo Stoakes (Oracle) > wrote: > > > > On Wed, Feb 25, 2026 at 08:25:04PM -0700, Nico Pache wrote: > > > Add three new mTHP statistics to track collapse failures for different > > > orders when encountering swap PTEs, excessive none PTEs, and shared PTEs: > > > > > > - collapse_exceed_swap_pte: Increment when mTHP collapse fails due to swap > > > PTEs > > > > > > - collapse_exceed_none_pte: Counts when mTHP collapse fails due to > > > exceeding the none PTE threshold for the given order > > > > > > - collapse_exceed_shared_pte: Counts when mTHP collapse fails due to shared > > > PTEs > > > > > > These statistics complement the existing THP_SCAN_EXCEED_* events by > > > providing per-order granularity for mTHP collapse attempts. The stats are > > > exposed via sysfs under > > > `/sys/kernel/mm/transparent_hugepage/hugepages-*/stats/` for each > > > supported hugepage size. > > > > > > As we currently dont support collapsing mTHPs that contain a swap or > > > shared entry, those statistics keep track of how often we are > > > encountering failed mTHP collapses due to these restrictions. > > > > > > Reviewed-by: Baolin Wang > > > Signed-off-by: Nico Pache > > > --- > > > Documentation/admin-guide/mm/transhuge.rst | 24 ++++++++++++++++++++++ > > > include/linux/huge_mm.h | 3 +++ > > > mm/huge_memory.c | 7 +++++++ > > > mm/khugepaged.c | 16 ++++++++++++--- > > > 4 files changed, 47 insertions(+), 3 deletions(-) > > > > > > diff --git a/Documentation/admin-guide/mm/transhuge.rst b/Documentation/admin-guide/mm/transhuge.rst > > > index c51932e6275d..eebb1f6bbc6c 100644 > > > --- a/Documentation/admin-guide/mm/transhuge.rst > > > +++ b/Documentation/admin-guide/mm/transhuge.rst > > > @@ -714,6 +714,30 @@ nr_anon_partially_mapped > > > an anonymous THP as "partially mapped" and count it here, even though it > > > is not actually partially mapped anymore. > > > > > > +collapse_exceed_none_pte > > > + The number of collapse attempts that failed due to exceeding the > > > + max_ptes_none threshold. For mTHP collapse, Currently only max_ptes_none > > > + values of 0 and (HPAGE_PMD_NR - 1) are supported. Any other value will > > > + emit a warning and no mTHP collapse will be attempted. khugepaged will > > > > It's weird to document this here but not elsewhere in the document? I mean I > > made this comment on the documentation patch also. > > I can add some more documentation but TBH I don't really know where or > what else to put. I checked a few of these other per-mTHP stats, and > none are referenced elsewhere. if anything these 3 additions are the > best documented ones. > > > > > Not sure if I missed you adding it to another bit of the docs? :) > > > > > + try to collapse to the largest enabled (m)THP size; if it fails, it will > > > + try the next lower enabled mTHP size. This counter records the number of > > > + times a collapse attempt was skipped for exceeding the max_ptes_none > > > + threshold, and khugepaged will move on to the next available mTHP size. > > > + > > > +collapse_exceed_swap_pte > > > + The number of anonymous mTHP PTE ranges which were unable to collapse due > > > + to containing at least one swap PTE. Currently khugepaged does not > > > + support collapsing mTHP regions that contain a swap PTE. This counter can > > > + be used to monitor the number of khugepaged mTHP collapses that failed > > > + due to the presence of a swap PTE. > > > + > > > +collapse_exceed_shared_pte > > > + The number of anonymous mTHP PTE ranges which were unable to collapse due > > > + to containing at least one shared PTE. Currently khugepaged does not > > > + support collapsing mTHP PTE ranges that contain a shared PTE. This > > > + counter can be used to monitor the number of khugepaged mTHP collapses > > > + that failed due to the presence of a shared PTE. > > > > All of these talk about 'ranges' that could be of any size. Are these useful > > metrics? Counting a bunch of failures and not knowing if they are 256 KB > > failures or 16 KB failures or whatever is maybe not so useful information? > > These are per-mTHP size statistics. If you look at the surrounding > examples and docs this all makes more sense. > > > > > Also, from the code, aren't you treating PMD events the same as mTHP ones from > > the point of view of these counters? Maybe worth documenting that? > > IIUC, yes but that is true of all these > > ``` > In /sys/kernel/mm/transparent_hugepage/hugepages-kB/stats, There are > also individual counters for each huge page size, which can be utilized to > monitor the system's effectiveness in providing huge pages for usage. Each > counter has its own corresponding file. > ``` > > > > > > + > > > As the system ages, allocating huge pages may be expensive as the > > > system uses memory compaction to copy data around memory to free a > > > huge page for use. There are some counters in ``/proc/vmstat`` to help > > > diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h > > > index 9941fc6d7bd8..e8777bb2347d 100644 > > > --- a/include/linux/huge_mm.h > > > +++ b/include/linux/huge_mm.h > > > @@ -144,6 +144,9 @@ enum mthp_stat_item { > > > MTHP_STAT_SPLIT_DEFERRED, > > > MTHP_STAT_NR_ANON, > > > MTHP_STAT_NR_ANON_PARTIALLY_MAPPED, > > > + MTHP_STAT_COLLAPSE_EXCEED_SWAP, > > > + MTHP_STAT_COLLAPSE_EXCEED_NONE, > > > + MTHP_STAT_COLLAPSE_EXCEED_SHARED, > > > __MTHP_STAT_COUNT > > > }; > > > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > > index 228f35e962b9..1049a207a257 100644 > > > --- a/mm/huge_memory.c > > > +++ b/mm/huge_memory.c > > > @@ -642,6 +642,10 @@ DEFINE_MTHP_STAT_ATTR(split_failed, MTHP_STAT_SPLIT_FAILED); > > > DEFINE_MTHP_STAT_ATTR(split_deferred, MTHP_STAT_SPLIT_DEFERRED); > > > DEFINE_MTHP_STAT_ATTR(nr_anon, MTHP_STAT_NR_ANON); > > > DEFINE_MTHP_STAT_ATTR(nr_anon_partially_mapped, MTHP_STAT_NR_ANON_PARTIALLY_MAPPED); > > > +DEFINE_MTHP_STAT_ATTR(collapse_exceed_swap_pte, MTHP_STAT_COLLAPSE_EXCEED_SWAP); > > > +DEFINE_MTHP_STAT_ATTR(collapse_exceed_none_pte, MTHP_STAT_COLLAPSE_EXCEED_NONE); > > > +DEFINE_MTHP_STAT_ATTR(collapse_exceed_shared_pte, MTHP_STAT_COLLAPSE_EXCEED_SHARED); > > > > Is there a reason there's such a difference between the names and the actual > > enum names? > > Good point I didnt think about that. I can update those as long as > they don't conflict with something else (I forget why i named them > like this). > > > > > > + > > > > > > static struct attribute *anon_stats_attrs[] = { > > > &anon_fault_alloc_attr.attr, > > > @@ -658,6 +662,9 @@ static struct attribute *anon_stats_attrs[] = { > > > &split_deferred_attr.attr, > > > &nr_anon_attr.attr, > > > &nr_anon_partially_mapped_attr.attr, > > > + &collapse_exceed_swap_pte_attr.attr, > > > + &collapse_exceed_none_pte_attr.attr, > > > + &collapse_exceed_shared_pte_attr.attr, > > > NULL, > > > }; > > > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > > index c739f26dd61e..a6cf90e09e4a 100644 > > > --- a/mm/khugepaged.c > > > +++ b/mm/khugepaged.c > > > @@ -595,7 +595,9 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, > > > continue; > > > } else { > > > result = SCAN_EXCEED_NONE_PTE; > > > - count_vm_event(THP_SCAN_EXCEED_NONE_PTE); > > > + if (is_pmd_order(order)) > > > + count_vm_event(THP_SCAN_EXCEED_NONE_PTE); > > > + count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_NONE); > > > > It's a bit gross to have separate stats for both thp and mthp but maybe > > unavoidable from a legacy stand point. > > I agree but that's how it currently is. Perhaps we can add this to the > TODO list for THP work. > > > > > Why are we dropping the _PTE suffix? > > I follow the convention that the other mTHP stats follow for example > (MTHP_STAT_SPLIT_DEFERRED) > > > > > > goto out; > > > } > > > } > > > @@ -631,10 +633,17 @@ static enum scan_result __collapse_huge_page_isolate(struct vm_area_struct *vma, > > > * shared may cause a future higher order collapse on a > > > * rescan of the same range. > > > */ > > > - if (!is_pmd_order(order) || (cc->is_khugepaged && > > > - shared > khugepaged_max_ptes_shared)) { > > > > OK losing track here :) as the series sadly doesn't currently apply so can't > > browser file as is. > > > > In the code I'm looking at, there's also a ++shared here that I guess another > > patch removed? > > > > Is this in the folio_maybe_mapped_shared() branch? > > yes the counting is now done at the top of that branch. > > > > > > + if (!is_pmd_order(order)) { > > > + result = SCAN_EXCEED_SHARED_PTE; > > > + count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED); > > > + goto out; > > > + } > > > + > > > + if (cc->is_khugepaged && > > > + shared > khugepaged_max_ptes_shared) { > > > result = SCAN_EXCEED_SHARED_PTE; > > > count_vm_event(THP_SCAN_EXCEED_SHARED_PTE); > > > + count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SHARED); > > > goto out; > > > > Anyway I'm a bit lost on this logic until a respin but this looks like a LOT of > > code duplication. I see David alluded to a refactoring so maybe what he suggests > > will help (not had a chance to check what it is specifically :P) > > Yep :) should look cleaner in the next one. Although it's quite a bit > of refactoring. I'll be praying that i got it right on the first go, > and I put all the other pieces in the desired spot. > > > > > > } > > > } > > > @@ -1081,6 +1090,7 @@ static enum scan_result __collapse_huge_page_swapin(struct mm_struct *mm, > > > * range. > > > */ > > > if (!is_pmd_order(order)) { > > > + count_mthp_stat(order, MTHP_STAT_COLLAPSE_EXCEED_SWAP); > > > > Hmm I thought we were incrementing mthp stats for pmd sized also? > > Yes we are supposed to. I've already refactored and it looks fine > there... perhaps i missed this one in this version! > > Cheers, > > -- Nico > > > > > > pte_unmap(pte); > > > mmap_read_unlock(mm); > > > result = SCAN_EXCEED_SWAP_PTE; > > > -- > > > 2.53.0 > > > > > > > Cheers, Lorenzo > > >