On Tue, Oct 20, 2020 at 12:21:38PM +0100, Matthew Wilcox wrote: > On Tue, Oct 20, 2020 at 03:59:28PM +1100, Dave Chinner wrote: > > On Tue, Oct 20, 2020 at 02:43:57AM +0100, Matthew Wilcox wrote: > > > This is a weird one ... which is good because it means the obvious > > > ones have been fixed and now I'm just tripping over the weird cases. > > > And fortunately, xfstests exercises the weird cases. > > > > > > 1. The file is 0x3d000 bytes long. > > > 2. A readahead allocates an order-2 THP for 0x3c000-0x3ffff > > > 3. We simulate a read error for 0x3c000-0x3cfff > > > 4. Userspace writes to 0x3d697 to 0x3dfaa > > > > So this is a write() beyond EOF, yes? > > > > If yes, then we first go through this path: > > > > xfs_file_buffered_aio_write() > > xfs_file_aio_write_checks() > > iomap_zero_range(isize, pos - isize) > > > > To zero the region between the current EOF and where the new write > > starts. i.e. from 0x3d000 to 0x3d696. > > Yes. That calls iomap_write_begin() which calls iomap_split_page() > which is where we run into trouble. I elided the exact path from the > description of the problem. > > > > 5. iomap_write_begin() gets the 0x3c page, sees it's THP and !Uptodate > > > so it calls iomap_split_page() (passing page 0x3d) > > > > Splitting the page because it's !Uptodate seems rather drastic to > > me. Why does it need to split the page here? > > Because we can't handle Dirty, !Uptodate THPs in the truncate path. > Previous discussion: > https://lore.kernel.org/linux-mm/20200821144021.GV17456@xxxxxxxxxxxxxxxxxxxx/ Maybe I'm just dense, but this doesn't explain the reason for needing to split THPs during partial THP invalidation, nor the reason why we need to split THPs when the write path sees a partially up to date THP. iomap is supposed to be tracking the sub-page regions that are not up to date, so why would we ever need to split the page to get sub-page regions into the correct state? i.e. why do THPs need to be handled any differently to a block size < page size situation with a normal PAGE_SIZE page? FWIW, didn't you change the dirty tracking to be done sub-page and held in the iomap_page? If so, releasing the iomap_page on a partial page invalidation looks ... problematic. i.e. not only are you throwing away the per-block up-to-date state on a THP, you're alos throwing away the per-block dirty state. i.e. we still need that per-block state to be maintained once the THP has been split - the split pages should be set to the correct state held on the THP as the split progresses. IOWs, I suspect that split_huge_page() needs to call into iomap to determine the actual state of each individual sub-page from the iomap_page state attached to the huge page. > The current assumption is that a !Uptodate THP is due to a read error, > and so the sensible thing to do is split it and handle read errors at > a single-page level. Why? Apart from the range of the file coverd by the page, how is handling a read error at a single page level any different from handling it at a THP level? Alternatively, if there's a read error on THP-based readahead, then why isn't the entire THP tossed away when a subsequent read sees PageError() so it can then be re-read synchronously into the cache using single pages? > I've been playing around with creating THPs in the write path, and that > offers a different pathway to creating Dirty, !Uptodate THPs, so this > may also change at some point. I'd like to get what I have merged and > then figure out how to make this better. Sure. However, again, splitting THPs in this situation makes no sense to me. I can't see why we don't just treat them like a normal page and the sub-page !uptodate range filling algorithms just do their normal work on them... > > Also, this concerns me: if we are exposing the cached EOF page via > > mmap, it needs to contain only zeroes in the region beyond EOF so > > that we don't expose stale data to userspace. Hence when a THP that > > contains EOF is instantiated, we have to ensure that the region > > beyond EOF is compeltely zeroed. It then follows that if we read all > > the data in that THP up to EOF, then the page is actually up to > > date... > > We do that in iomap_readpage_actor(). Had the readahead I/O not "failed", > we'd've had an Uptodate THP which straddled EOF. If the IO error in a THP is the trigger for bad things here, then surely the correct thing to do is trash the THP on IO error, not leave a landmine that every path has to handle specially... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx
