On Tue, Jan 07, 2025 at 11:24:13AM -0700, Jens Axboe wrote: > On 1/7/25 11:19 AM, Darrick J. Wong wrote: > > On Tue, Jan 07, 2025 at 09:05:15AM -0700, Jens Axboe wrote: > >> Using RWF_DONTCACHE tells the kernel that any page cache instantiated > >> by this operation should get pruned once the operation completes. If > >> data is in cache prior to the operation it will remain there. > >> > >> Add ops for testing both the read and write side of this. At startup, > >> kernel support for this feature is probed. If support isn't available, > >> uncached/dontcache IO is performed as regular buffered IO. If -Z is > >> used to turn on O_DIRECT, then uncached/dontcache IO isn't performed. > > > > Huh. Does the kernel reject RWF_DONTCACHE for directio? And, if a > > It doesn't, it simply ignores it. Not sure why you ask? It's buffered IO > after all, falling back to just clearing the flag seems like the most > sensible solution here. I was curious, because your code does has_dontcache=0 when -Z is used to select directio mode. So I wondered if it that was because the kernel would return EOPNOTSUPP for directio + RWF_DONTCACHE? :) Then I wondered if there was actually a good usecase either for letting userspace specify it, or for filesystems to add it for buffered write fallback. At this point I would wager there's a stronger case for adding drop-behind automatically because userspace shouldn't have to communicate "write this without accessing the page cache, and don't leave file contents in the page cache that I already told you not to do." Anyway the fstests change satisfies me now so Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx> --D > > directio implementation falls back to the pagecache (e.g. xfs when doing > > a sub-fsblock cow write), do we: > > > > (a) want RWF_DONTCACHE to propagate through to the buffered io > > implementation (which I think xfs does) and > > Maybe? The current implementation keeps things simple and doesn't touch > any of that stuff, but conceptually it'd make sense to mark those > buffered ranges as uncached, if instantiated as buffered IO on behalf of > direct IO. > > > (b) should filesystems *turn it on* any time they fall back, even if the > > original IO request didn't set DONTCACHE? > > Same answer :-) > > -- > Jens Axboe >
