Re: [PATCHSET v4] Uncached buffered IO

[Jens Axboe <axboe@kernel.dk>]

On 11/11/24 7:08 AM, Jens Axboe wrote:
> On 11/11/24 5:55 AM, Stefan Metzmacher wrote:
>> Hi Jens,
>>
>> I'm wondering about the impact on memory mapped files.
>>
>> Let's say one (or more) process(es) called mmap on a file in order to
>> use the content of the file as persistent shared memory.
>> As far as I understand pages from the page cache are used for this.
>>
>> Now another process uses RWF_UNCACHED for a read of the same file.
>> What happens if the pages are removed from the page cache?
>> Or is the removal deferred based on some refcount?
> 
> For mmap, if a given page isn't in page cache, it'll get faulted in.
> Should be fine to have mmap and uncached IO co-exist. If an uncached
> read IO instantiates a page, it'll get reaped when the data has been
> copied. If an uncached IO hits an already existing page (eg mmap faulted
> it in), then it won't get touched. Same thing happens with mixing
> buffered and uncached IO. The latter will only reap parts it
> instantiated to satisfy the operation. That doesn't matter in terms of
> data integrity, only in terms of the policy of uncached leaving things
> alone it didn't create to satisfy the operation.
> 
> This is really no different than say using mmap and evicting pages, they
> will just get faulted in if needed.

Turns out that was nonsense, as per Kiril's comments on the other thread.
For pages that are actually mapped, we'll have to skip the invalidation
as it's not safe to do so.

-- 
Jens Axboe