On Wed, Aug 11, 2021 at 03:04:07PM -0400, Jeff Layton wrote: > On Mon, 2021-08-09 at 16:30 +0100, Matthew Wilcox wrote: > > On Mon, Aug 09, 2021 at 03:48:56PM +0100, Christoph Hellwig wrote: > > > On Tue, Aug 03, 2021 at 04:28:14PM +0100, Matthew Wilcox wrote: > > > > Solution 1: Add an array of dirty bits to the iomap_page > > > > data structure. This patch already exists; would need > > > > to be adjusted slightly to apply to the current tree. > > > > https://lore.kernel.org/linux-xfs/7fb4bb5a-adc7-5914-3aae-179dd8f3adb1@xxxxxxxxxx/ > > > > > > > Solution 2a: Replace the array of uptodate bits with an array of > > > > dirty bits. It is not often useful to know which parts of the page are > > > > uptodate; usually the entire page is uptodate. We can actually use the > > > > dirty bits for the same purpose as uptodate bits; if a block is dirty, it > > > > is definitely uptodate. If a block is !dirty, and the page is !uptodate, > > > > the block may or may not be uptodate, but it can be safely re-read from > > > > storage without losing any data. > > > > > > 1 or 2a seems like something we should do once we have lage folio > > > support. > > > > > > > > > > Solution 2b: Lose the concept of partially uptodate pages. If we're > > > > going to write to a partial page, just bring the entire page uptodate > > > > first, then write to it. It's not clear to me that partially-uptodate > > > > pages are really useful. I don't know of any network filesystems that > > > > support partially-uptodate pages, for example. It seems to have been > > > > something we did for buffer_head based filesystems "because we could" > > > > rather than finding a workload that actually cares. > > > > > I may be wrong, but I thought NFS actually could deal with partially > uptodate pages. In some cases it can opt to just do a write to a page > w/o reading first and flush just that section when the time comes. > > I think the heuristics are in nfs_want_read_modify_write(). #3 may be a > better way though. Yes; I was talking specifically about iomap here. I like what NFS does; it makes a lot of sense to me. The only thing I'd like to change for NFS is to add an ->is_partially_uptodate implementation. But that only matters if somebody calls read() on some bytes they just wrote. And I don't know that that's a particularly common or interesting thing to do. I suppose if there are two processes with one writing to the file and the other reading from it, we might get that. > > > The uptodate bit is important for the use case of a smaller than page > > > size buffered write into a page that hasn't been read in already, which > > > is fairly common for things like log writes. So I'd hate to lose this > > > optimization. > > > > > > > (it occurs to me that solution 3 actually allows us to do IOs at storage > > > > block size instead of filesystem block size, potentially reducing write > > > > amplification even more, although we will need to be a bit careful if > > > > we're doing a CoW.) > > > > > > number 3 might be nice optimization. The even better version would > > > be a disk format change to just log those updates in the log and > > > otherwise use the normal dirty mechanism. I once had a crude prototype > > > for that. > > > > That's a bit beyond my scope at this point. I'm currently working on > > write-through. Once I have that working, I think the next step is: > > > > - Replace the ->uptodate array with a ->dirty array > > - If the entire page is Uptodate, drop the iomap_page. That means that > > writebacks will write back the entire folio, not just the dirty > > pieces. > > - If doing a partial page write > > - If the write is block-aligned (offset & length), leave the page > > !Uptodate and mark the dirty blocks > > - Otherwise bring the entire page Uptodate first, then mark it dirty > > > > To take an example of a 512-byte block size file accepting a 520 byte > > write at offset 500, we currently submit two reads, one for bytes 0-511 > > and the second for 1024-1535. We're better off submitting a read for > > bytes 0-4095 and then overwriting the entire thing. > > > > But it's still better to do no reads at all if someone submits a write > > for bytes 512-1023, or 512-N where N is past EOF. And I'd preserve > > that behaviour. > > I like this idea too. > > I'd also point out that both cifs and ceph (at least) can read and write > "around" the cache in some cases (using non-pagecache pages) when they > can't get the proper oplock/lease/caps from the server. Both of them > have completely separate "uncached" codepaths, that are distinct from > the O_DIRECT cases. > > This scheme could potentially be a saner method of dealing with those > situations too. Possibly ... how do cifs & ceph handle writable mmap() when they can't get the proper exclusive access from the server? The major downside I can see is that if you have multiple writes to the same page then the writes will be serialised. But maybe they already are by the scheme used by cifs & ceph? (That could be worked around by tracking outstanding writes to the server and only clearing the writeback bit once all writes have completed)
