On Tue, Jun 13, 2023 at 08:49:05AM +1000, Dave Chinner wrote: > On Mon, Jun 12, 2023 at 09:39:08PM +0100, Matthew Wilcox (Oracle) wrote: > > Allow callers of __filemap_get_folio() to specify a preferred folio > > order in the FGP flags. This is only honoured in the FGP_CREATE path; > > if there is already a folio in the page cache that covers the index, > > we will return it, no matter what its order is. No create-around is > > attempted; we will only create folios which start at the specified index. > > Unmodified callers will continue to allocate order 0 folios. > ..... > > - /* Init accessed so avoid atomic mark_page_accessed later */ > > - if (fgp_flags & FGP_ACCESSED) > > - __folio_set_referenced(folio); > > + if (!mapping_large_folio_support(mapping)) > > + order = 0; > > + if (order > MAX_PAGECACHE_ORDER) > > + order = MAX_PAGECACHE_ORDER; > > + /* If we're not aligned, allocate a smaller folio */ > > + if (index & ((1UL << order) - 1)) > > + order = __ffs(index); > > If I read this right, if we pass in an unaligned index, we won't get > the size of the folio we ask for? Right. That's implied by (but perhaps not obvious from) the changelog. Folios are always naturally aligned in the file, so an order-4 folio has to start at a multiple of 16. If the index you pass in is not a multiple of 16, we can't create an order-4 folio without starting at an earlier index. For a 4kB block size filesystem, that's what we want. Applications _generally_ don't write backwards, so creating an order-4 folio is just wasting memory. > e.g. if we want an order-4 folio (64kB) because we have a 64kB block > size in the filesystem, then we have to pass in an index that > order-4 aligned, yes? > > I ask this, because the later iomap code that asks for large folios > only passes in "pos >> PAGE_SHIFT" so it looks to me like it won't > allocate large folios for anything other than large folio aligned > writes, even if we need them. > > What am I missing? Perhaps what you're missing is that this isn't trying to solve the problem of supporting a bs > ps filesystem? That's also a worthwhile project, but it's not this project. In fact, I'd say that project is almost orthogonal to this one; for this usage we can always fall back to smaller folios on memory pressure or misalignment. For a bs > ps block device, we have to allocate folios at least as large as the blocksize and cannot fall back to smaller folios. For a bs > ps filesystem on a bdev with bs == ps, we can fall back (as your prototype showed).
