On Tue, Jun 13, 2023 at 11:30:13AM +1000, Dave Chinner wrote: > 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. OK. Bringing up the 64kB block size filesystem confused me. > 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? Right. > 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? Yes. Assuming we get another write ... > 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... At this point we're arguing about what I/O pattern to optimise for. I'm going for a "do no harm" approach where we only allocate exactly as much memory as we did before. You're advocating for a higher-risk/higher-reward approach. I'd prefer the low-risk approach for now; we can change it later! I'd like to see some amount of per-fd write history (as we have per-fd readahead history) to decide whether to allocate large folios ahead of the current write position. As with readahead, I'd like to see that even doing single-byte writes can result in the allocation of large folios, as long as the app has done enough of them.
