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    SubjectRe: [PATCH] Re: Bio device too big | kernel BUG at mm/filemap.c:537!
    On Tuesday February 6, akpm@linux-foundation.org wrote:
    > On Wed, 7 Feb 2007 10:26:56 +1100
    > Neil Brown <neilb@suse.de> wrote:
    >
    > > +static int bio_fits_rdev(struct bio *bi)
    > > +{
    > > + request_queue_t *q = bdev_get_queue(bi->bi_bdev);
    > > +
    > > + if ((bi->bi_size>>9) > q->max_sectors)
    > > + return 0;
    > > + blk_recount_segments(q, bi);
    > > + if (bi->bi_phys_segments > q->max_phys_segments ||
    > > + bi->bi_hw_segments > q->max_hw_segments)
    > > + return 0;
    > > +
    > > + if (q->merge_bvec_fn)
    > > + /* it's too hard to apply the merge_bvec_fn at this stage,
    > > + * just just give up
    > > + */
    > > + return 0;
    > > +
    > > + return 1;
    > > +}
    >
    > Isn't think going to return 0 rather a lot of the time?

    Why do you say that?

    merge_bvec_fn is only set for dm, md, pktcdvd.c so what won't cause a
    zero in real-world cases (it rarely makes sense to put a raid5 on top
    of those things).
    So it will only return zero when ->max_sectors or ->max_*_segments are
    less than the default, and while there are certainly quite a few of
    those I wouldn't have expected them to be a majority (else we would
    have had more complaints from -mm testers).

    Yes, we should find the minimum of those values for devices currently
    in the array and use that information in raid5's merge_bvec_fn to stop
    bios growing too large. That would make it return 0 somewhat less
    often.
    But I think for a lot of modern hardware, it won't return 0 at all....
    or am I just not seeing some really obvious typo that you can see ??

    NeilBrown

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