linux-kernel - Re: [PATCHSET v4] Uncached buffered IO

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Message-ID: <f7a48046-30b9-4656-812a-f0579234eb5c@kernel.dk>
Date: Mon, 11 Nov 2024 16:54:25 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Stefan Metzmacher <metze@...ba.org>, linux-mm@...ck.org,
 linux-fsdevel@...r.kernel.org
Cc: hannes@...xchg.org, clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET v4] Uncached buffered IO

On 11/11/24 8:05 AM, Jens Axboe wrote:
> 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.

...and now v3 (just posted) actually does work like I described, it'll
co-exist with mmap.

-- 
Jens Axboe