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    SubjectRe: [PATCH 03/33] mm: Implement readahead_control pageset expansion
    On Wed, Feb 17, 2021 at 10:34:39PM +0000, David Howells wrote:
    > Matthew Wilcox <willy@infradead.org> wrote:
    >
    > > We're defeating the ondemand_readahead() algorithm here. Let's suppose
    > > userspace is doing 64kB reads, the filesystem is OrangeFS which only
    > > wants to do 4MB reads, the page cache is initially empty and there's
    > > only one thread doing a sequential read. ondemand_readahead() calls
    > > get_init_ra_size() which tells it to allocate 128kB and set the async
    > > marker at 64kB. Then orangefs calls readahead_expand() to allocate the
    > > remainder of the 4MB. After the app has read the first 64kB, it comes
    > > back to read the next 64kB, sees the readahead marker and tries to trigger
    > > the next batch of readahead, but it's already present, so it does nothing
    > > (see page_cache_ra_unbounded() for what happens with pages present).
    >
    > It sounds like Christoph is right on the right track and the vm needs to ask
    > the filesystem (and by extension, the cache) before doing the allocation and
    > before setting the trigger flag. Then we don't need to call back into the vm
    > to expand the readahead.

    Doesn't work. You could read my reply to Christoph, or try to figure out
    how to get rid of
    https://evilpiepirate.org/git/bcachefs.git/tree/fs/bcachefs/fs-io.c#n742
    for yourself.

    > Also, there's Steve's request to try and keep at least two requests in flight
    > for CIFS/SMB at the same time to consider.

    That's not relevant to this problem.

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