On 11/8/24 1:07 PM, Matthew Wilcox wrote: > On Fri, Nov 08, 2024 at 12:49:58PM -0700, Jens Axboe wrote: >> On 11/8/24 12:26 PM, Jens Axboe wrote: >>> On 11/8/24 11:46 AM, Matthew Wilcox wrote: >>>> On Fri, Nov 08, 2024 at 10:43:34AM -0700, Jens Axboe wrote: >>>>> +++ b/fs/iomap/buffered-io.c >>>>> @@ -959,6 +959,8 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i) >>>>> } >>>>> if (iter->iomap.flags & IOMAP_F_STALE) >>>>> break; >>>>> + if (iter->flags & IOMAP_UNCACHED) >>>>> + folio_set_uncached(folio); >>>> >>>> This seems like it'd convert an existing page cache folio into being >>>> uncached? Is this just leftover from a previous version or is that a >>>> design decision you made? >>> >>> I'll see if we can improve that. Currently both the read and write side >>> do drop whatever it touches. We could feasibly just have it drop >>> newly instantiated pages - iow, uncached just won't create new persistent >>> folios, but it'll happily use the ones that are there already. >> >> Well that was nonsense on the read side, it deliberately only prunes >> entries that has uncached set. For the write side, this is a bit >> trickier. We'd essentially need to know if the folio populated by >> write_begin was found in the page cache, or create from new. Any way we >> can do that? One way is to change ->write_begin() so it takes a kiocb >> rather than a file, but that's an amount of churn I'd rather avoid! >> Maybe there's a way I'm just not seeing? > > Umm. We can solve it for iomap with a new FGP_UNCACHED flag and > checking IOMAP_UNCACHED in iomap_get_folio(). Not sure how we solve it > for other filesystems though. Any filesystem which uses FGP_NOWAIT has > _a_ solution, but eg btrfs will need to plumb through a third boolean > flag (or, more efficiently, just start passing FGP flags to > prepare_one_folio()). Yeah that's true, forgot we already have the IOMAP_UNCACHED flag there and it's available in creation. Thanks, I'll start with that. -- Jens Axboe
