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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B688710000C X-Stat-Signature: e8nzkxw3dsmwcmtbk7c4awgkjhuostnx X-HE-Tag: 1751457321-720802 X-HE-Meta: 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 9IfUEsek urnpm8We7eC/xYBltPsOUN4LpWavrren6TwQGuzlR7YldhYI/jpFlp6peSpVhR4krI1jcecAyGlTDQrLGF4p4JiFnPNWIDH3SqeAz3OMn/FJp9itZ9aeo3E13j3L9BrCp3S0RGJl61YfFV0Pp4jJqoSDFEmYHMj+dL7WgfF7tc0TOCMzXLCf3XzCpyUOfTB3TDyz6ZlbfVZ0S7BotRyPHcBN1xKbw8KY3i/AnTH3jC3iyOnGUvR7E80jIegF88mM1q/JUeGQ7PrMkF8Ehw1gD78Q3GJJSicf30o9SfhIqppXkRm9VbZGb6kzIhE52bjj1jrBwjuI+kFj5c+BoZHRadJ5PFGMuIRBU5aKJb9r3PFInP0550QPRYBGsowwfqq6hYVWZ+UAxtOOnmm2aekItH7eWH0EEYqSEKjBBiFQhZBMOsVw= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 02.07.25 13:38, David Hildenbrand wrote: > >>> So by mapping more in a single page fault, you end up increasing "RSS". >>> But I wouldn't >>> call that "expected". I rather suspect that nobody will really care :) >> >> But tmpfs is a little special here. It uses the 'huge=' option to >> control large folio allocation. So, I think users should know they want >> to use large folios and build the whole mapping for the large folios. >> That is why I call it 'expected'. > > Well, if your distribution decides to set huge= on /tmp or something > like that, your application might have very little saying in that, right? :) > > Again, I assume it's fine, but we might find surprises on the way. > >>> >>> The thing is, when you *allocate* a new folio, it must adhere at least to >>> pagecache alignment (e.g., cannot place an order-2 folio at pgoff 1) -- >> >> Yes, agree. >> >>> that is what >>> thp_vma_suitable_order() checks. Otherwise you cannot add it to the >>> pagecache. >> >> But this alignment is not done by thp_vma_suitable_order(). >> >> For tmpfs, it will check the alignment in shmem_suitable_orders() via: >> " >> if (!xa_find(&mapping->i_pages, &aligned_index, >> aligned_index + pages - 1, XA_PRESENT)) >> " > > That's not really alignment check, that's just checking whether a > suitable folio order spans already-present entries, no? > > Finding suitable orders is still up to other code IIUC. > >> >> For other fs systems, it will check the alignment in >> __filemap_get_folio() via: >> " >> /* If we're not aligned, allocate a smaller folio */ >> if (index & ((1UL << order) - 1)) >> order = __ffs(index); >> " >> >>> But once you *obtain* a folio from the pagecache and are supposed to map it >>> into the page tables, that must already hold true. >>> >>> So you should be able to just blindly map whatever is given to you here >>> AFAIKS. >>> >>> If you would get a pagecache folio that violates the linear page offset >>> requirement >>> at that point, something else would have messed up the pagecache. >> >> Yes. But the comments from thp_vma_suitable_order() is not about the >> pagecache alignment, it says "the order-aligned addresses in the VMA map >> to order-aligned offsets within the file", > > Let's dig, it's confusing. > > The code in question is: > > if (!IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff, > hpage_size >> PAGE_SHIFT)) > > So yes, I think this tells us: if we would have a PMD THP in the > pagecache, would we be able to map it with a PMD. If not, then don't > bother with allocating a PMD THP. > > Of course, this also applies to other orders, but for PMD THPs it's > probably most relevant: if we cannot even map it through a PMD, then > probably it could be a wasted THP. > > So yes, I agree: if we are both no missing something, then this > primarily relevant for the PMD case. > > And it's more about "optimization" than "correctness" I guess? Correction: only if a caller doesn't assume that this is an implicit pagecache alignment check. Not sure if that might be the case for shmem when it calls thp_vma_suitable_order() with a VMA ... -- Cheers, David / dhildenb