Re: [PATCH 05/15] mm: allow read-ahead with IOCB_NOWAIT set

[Jens Axboe]

On 6/23/20 7:46 PM, Matthew Wilcox wrote:
> On Wed, Jun 24, 2020 at 11:02:53AM +1000, Dave Chinner wrote:
>> On Thu, Jun 18, 2020 at 08:43:45AM -0600, Jens Axboe wrote:
>>> The read-ahead shouldn't block, so allow it to be done even if
>>> IOCB_NOWAIT is set in the kiocb.
>>
>> Doesn't think break preadv2(RWF_NOWAIT) semantics for on buffered
>> reads? i.e. this can now block on memory allocation for the page
>> cache, which is something RWF_NOWAIT IO should not do....
> 
> Yes.  This eventually ends up in page_cache_readahead_unbounded()
> which gets its gfp flags from readahead_gfp_mask(mapping).
> 
> I'd be quite happy to add a gfp_t to struct readahead_control.
> The other thing I've been looking into for other reasons is adding
> a memalloc_nowait_{save,restore}, which would avoid passing down
> the gfp_t.

That was my first thought, having the memalloc_foo_save/restore for
this. I don't think adding a gfp_t to readahead_control is going
to be super useful, seems like the kind of thing that should be
non-blocking by default.

-- 
Jens Axboe