Re: [PATCH v5 3/3] block: bio_iov_iter_get_pages: pin more pages for multi-segment IOs

[Ming Lei <ming.lei@redhat.com>]

On Wed, Aug 22, 2018 at 12:33:05PM +0200, Jan Kara wrote:
> On Wed 22-08-18 10:02:49, Martin Wilck wrote:
> > On Mon, 2018-07-30 at 20:37 +0800, Ming Lei wrote:
> > > On Wed, Jul 25, 2018 at 11:15:09PM +0200, Martin Wilck wrote:
> > > > 
> > > > +/**
> > > > + * bio_iov_iter_get_pages - pin user or kernel pages and add them
> > > > to a bio
> > > > + * @bio: bio to add pages to
> > > > + * @iter: iov iterator describing the region to be mapped
> > > > + *
> > > > + * Pins pages from *iter and appends them to @bio's bvec array.
> > > > The
> > > > + * pages will have to be released using put_page() when done.
> > > > + * The function tries, but does not guarantee, to pin as many
> > > > pages as
> > > > + * fit into the bio, or are requested in *iter, whatever is
> > > > smaller.
> > > > + * If MM encounters an error pinning the requested pages, it
> > > > stops.
> > > > + * Error is returned only if 0 pages could be pinned.
> > > > + */
> > > > +int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> > > > +{
> > > > +	unsigned short orig_vcnt = bio->bi_vcnt;
> > > > +
> > > > +	do {
> > > > +		int ret = __bio_iov_iter_get_pages(bio, iter);
> > > > +
> > > > +		if (unlikely(ret))
> > > > +			return bio->bi_vcnt > orig_vcnt ? 0 : ret;
> > > > +
> > > > +	} while (iov_iter_count(iter) && !bio_full(bio));
> > > 
> > > When 'ret' isn't zero, and some partial progress has been made, seems
> > > less pages
> > > might be obtained than requested too. Is that something we need to
> > > worry about?
> > 
> > This would be the case when VM isn't willing or able to fulfill the
> > page-pinning request. Previously, we came to the conclusion that VM has
> > the right to do so. This is the reason why callers have to check the
> > number of pages allocated, and either loop over
> > bio_iov_iter_get_pages(), or fall back to buffered I/O, until all pages
> > have been obtained. All callers except the blockdev fast path do the
> > former. 
> > 
> > We could add looping in __blkdev_direct_IO_simple() on top of the
> > current patch set, to avoid fallback to buffered IO in this corner
> > case. Should we? If yes, only for WRITEs, or for READs as well?
> > 
> > I haven't encountered this situation in my tests, and I'm unsure how to
> > provoke it - run a direct IO test under high memory pressure?
> 
> Currently, iov_iter_get_pages() is always guaranteed to get at least one
> page as that is current guarantee of get_user_pages() (unless we hit
> EFAULT obviously). So bio_iov_iter_get_pages() as is now is guaranteed to

Is it possible for this EFAULT to happen on the user-space VM?


Thanks,
Ming