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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 09.07.24 09:47, Ryan Roberts wrote: > On 08/07/2024 21:50, David Hildenbrand wrote: >> On 08.07.24 14:29, Ryan Roberts wrote: >>> On 08/07/2024 12:36, Barry Song wrote: >>>> On Mon, Jul 8, 2024 at 11:24 PM Ryan Roberts 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 also forgot most of the discussion, but these 3 legacy counters are really >> only (currently) incremented for shmem. I think my idea was to keep everything >> as FILE_ for now, maybe at some point make the pagecache also use them, and then >> maybe have separate FILE_ + SHMEM_. >> >> But yeah, likely it's best to only have "shmem" here for now, because who knows >> what we can actually change about the legacy counters. But it's always though >> messing with legacy stuff that is clearly suboptimal ... > > Sorry David, I've read your response a bunch of times and am still not > completely sure what you are advocating for. > > To make my proposal crystal clear, I think we should leave the legacy counters > alone - neither change their name nor what they count (which is _only_ shmem). I > think we should rename the new mTHP counters to "shmem" and have them continue > to only count shmem. Then additionally, I think we should introduce new "file" > mTHP counters that count the page cache allocations (that's a future set of > patches, which I'm working on together with user controls to determine which > mTHP sizes can be used by page cache). > > As suggested by Barry, I propose to also improve the documentation for the > legacy counters to make it clear that dispite their name being "file" they are > actually counting "shmem". I'll do this for v2. Yes, and please. Likely we should document for the legacy ones (if not already done) that they only track PMD-sized THPs. > > David, would you support this approach? If so, I'd like to push this forwards > asap so that it gets into v6.11 to avoid ever exposing the mthp counters with > the "file" name. Yes, sorry for not being clear. -- Cheers, David / dhildenb