mm: use limited read-ahead to satisfy read

For the case where read-ahead is disabled on the file, or if the cgroup
is congested, ensure that we can at least do 1 page of read-ahead to
make progress on the read in an async fashion. This could potentially be
larger, but it's not needed in terms of functionality, so let's error on
the side of caution as larger counts of pages may run into reclaim
issues (particularly if we're congested).

This makes sure we're not hitting the potentially sync ->readpage() path
for IO that is marked IOCB_WAITQ, which could cause us to block. It also
means we'll use the same path for IO, regardless of whether or not
read-ahead happens to be disabled on the lower level device.

Acked-by: Johannes Weiner <[email protected]>
Reported-by: Matthew Wilcox (Oracle) <[email protected]>
Reported-by: Hao_Xu <[email protected]>
[axboe: updated for new ractl API]
Signed-off-by: Jens Axboe <[email protected]>

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