On Mon, Aug 14, 2023 at 12:06 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Mon, Aug 14, 2023 at 11:47:50AM -0700, Zach O'Keefe wrote: > > Willy -- I'm not up-to-date on what is happening on the THP-fs front. > > Should we be checking for a ->huge_fault handler here? > > Oh, thank goodness, I thought you were cc'ing me to ask a DAX question ... :) > 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 indices 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 doing 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. Saurabh, perhaps you can elaborate on your use case a bit more, and how that anonymous check broke you? Best, Zach
