On Tue, Jun 13, 2023 at 01:42:51AM +0100, Matthew Wilcox wrote: > 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? No, that's not what I'm asking about. I know there's other changes needed to enforce minimum folio size/alignment for bs > ps. What I'm asking about is when someone does a 16kB write at offset 12kB, they won't get a large folio allocated at all, right? Even though the write is large enough to enable it? Indeed, if we do a 1MB write at offset 4KB, we'll get 4kB at 4KB, 8KB and 12kB (because we can't do order-1 folios), then order-2 at 16KB, order-3 at 32kB, and so on until we hit offset 1MB where we will do an order-0 folio allocation again (because the remaining length is 4KB). The next 1MB write will then follow the same pattern, right? I think this ends up being sub-optimal and fairly non-obvious non-obvious behaviour from the iomap side of the fence which is clearly asking for high-order folios to be allocated. i.e. a small amount of allocate-around to naturally align large folios when the page cache is otherwise empty would make a big difference to the efficiency of non-large-folio-aligned sequential writes... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
