On Thu, Aug 05, 2021 at 05:35:33PM +0100, David Howells wrote: > With Willy's upcoming folio changes, from a filesystem point of view, we're > going to be looking at folios instead of pages, where: > > - a folio is a contiguous collection of pages; > > - each page in the folio might be standard PAGE_SIZE page (4K or 64K, say) or > a huge pages (say 2M each); This is not a great way to explain folios. If you're familiar with compound pages, a folio is a new type for either a base page or the head page of a compound page; nothing more and nothing less. If you're not familiar with compound pages, a folio contains 2^n contiguous pages. They are treated as a single unit. > - a folio has one dirty flag and one writeback flag that applies to all > constituent pages; > > - a complete folio currently is limited to PMD_SIZE or order 8, but could > theoretically go up to about 2GiB before various integer fields have to be > modified (not to mention the memory allocator). Filesystems should not make an assumption about this ... I suspect the optimum page size scales with I/O bandwidth; taking PCI bandwidth as a reasonable proxy, it's doubled five times in twenty years. > Willy is arguing that network filesystems should, except in certain very > special situations (eg. O_SYNC), only write whole folios (limited to EOF). I did also say that the write could be limited by, eg, a byte-range lease on the file. If the client doesn't have permission to write a byte range, then it doesn't need to write it back.
