On 1/7/25 4:22 PM, Darrick J. Wong wrote: > 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? :) Ah gotcha - no that's not the case, it's just that it doesn't make any sense to open a file O_DIRECT and then use RWF_DONTCACHE, when it already shouldn't be cached. Outside of the case you brought up where we'd want to drop page cache for O_DIRECT that ends up allocating page cache, but that should be done regardless imho. > 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." Yeah agree, I think we should just use the same mechanism for O_DIRECT instantiated page cache, without needing the app setting RWF_DONTCACHE. O_DIRECT is also slowed down by the existence of page cache, which can be annoying particularly when you fill a file with O_DIRECT and now have a bunch of page cache for it. > Anyway the fstests change satisfies me now so > Reviewed-by: "Darrick J. Wong" <djwong@xxxxxxxxxx> Thanks! -- Jens Axboe
