Re: [PATCH 2/2] fsx: add support for RWF_DONTCACHE

[Jens Axboe <axboe@kernel.dk>]

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.

> 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