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Dan Williams > ; linux-mm@kvack.org; Yang Shi > ; linux-kernel@vger.kernel.org > Subject: Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill > __transhuge_page_enabled()" >=20 > On Mon, Aug 14, 2023 at 05:04:47PM -0700, Zach O'Keefe wrote: > > > From a large folios perspective, filesystems do not implement a > > > special handler. They call filemap_fault() (directly or indirectly) > > > from their > > > ->fault handler. If there is already a folio in the page cache > > > ->which > > > satisfies this fault, we insert it into the page tables (no matter > > > what size it is). If there is no folio, we call readahead to > > > populate that index in the page cache, and probably some other indice= s > around it. > > > That's do_sync_mmap_readahead(). > > > > > > If you look at that, you'll see that we check the VM_HUGEPAGE flag, > > > and if set we align to a PMD boundary and read two PMD-size pages > > > (so that we can do async readahead for the second page, if we're doin= g a > linear scan). > > > If the VM_HUGEPAGE flag isn't set, we'll use the readahead algorithm > > > to decide how large the folio should be that we're reading into; if > > > it's a random read workload, we'll stick to order-0 pages, but if > > > we're getting good hit rate from the linear scan, we'll increase the > > > size (although we won't go past PMD size) > > > > > > There's also the ->map_pages() optimisation which handles page > > > faults locklessly, and will fail back to ->fault() if there's even a > > > light breeze. I don't think that's of any particular use in > > > answering your question, so I'm not going into details about it. > > > > > > I'm not sure I understand the code that's being modified well enough > > > to be able to give you a straight answer to your question, but > > > hopefully this is helpful to you. > > > > Thank you, this was great info. I had thought, incorrectly, that large > > folio work would eventually tie into that ->huge_fault() handler > > (should be dax_huge_fault() ?) > > > > If that's the case, then faulting file-backed, non-DAX memory as > > (pmd-mapped-)THPs isn't supported at all, and no fault lies with the > > aforementioned patches. >=20 > Ah, wait, hang on. You absolutely can get a PMD mapping by calling into > ->fault. Look at how finish_fault() works: >=20 > if (pmd_none(*vmf->pmd)) { > if (PageTransCompound(page)) { > ret =3D do_set_pmd(vmf, page); > if (ret !=3D VM_FAULT_FALLBACK) > return ret; > } >=20 > if (vmf->prealloc_pte) > pmd_install(vma->vm_mm, vmf->pmd, &vmf->prealloc_= pte); >=20 > So if we find a large folio that is PMD mappable, and there's nothing at = vmf- > >pmd, we install a PMD-sized mapping at that spot. If that fails, we ins= tall the > preallocated PTE table at vmf->pmd and continue to trying set one or more > PTEs to satisfy this page fault. >=20 > So why, you may be asking, do we have ->huge_fault. Well, you should ask > the clown who did commit b96375f74a6d ... in fairness to me, > finish_fault() did not exist at the time, and the ability to return a PMD= -sized > page was added later. Do you think we can restore this earlier behaviour of kernel to allow page = fault for huge pages via ->huge_fault.