On Wed, Jun 20, 2018 at 10:32:53AM -0400, Brian Foster wrote:
> Sending again without the attachment... Christoph, let me know if it
> didn't hit your mbox at least.
>
> On Wed, Jun 20, 2018 at 09:56:55AM +0200, Christoph Hellwig wrote:
> > On Tue, Jun 19, 2018 at 12:52:11PM -0400, Brian Foster wrote:
> > > > + /*
> > > > + * Move the caller beyond our range so that it keeps making progress.
> > > > + * For that we have to include any leading non-uptodate ranges, but
> > >
> > > Do you mean "leading uptodate ranges" here? E.g., pos is pushed forward
> > > past those ranges we don't have to read, so (pos - orig_pos) reflects
> > > the initial uptodate range while plen reflects the length we have to
> > > read..?
> >
> > Yes.
> >
> > > > +
> > > > + do {
> > >
> > > Kind of a nit, but this catches my eye and manages to confuse me every
> > > time I look at it. A comment along the lines of:
> > >
> > > /*
> > > * Pass in the block aligned start/end so we get back block
> > > * aligned/adjusted poff/plen and can compare with unaligned
> > > * from/to below.
> > > */
> > >
> > > ... would be nice here, IMO.
> >
> > Fine with me.
> >
> > > > + iomap_adjust_read_range(inode, iop, &block_start,
> > > > + block_end - block_start, &poff, &plen);
> > > > + if (plen == 0)
> > > > + break;
> > > > +
> > > > + if ((from > poff && from < poff + plen) ||
> > > > + (to > poff && to < poff + plen)) {
> > > > + status = iomap_read_page_sync(inode, block_start, page,
> > > > + poff, plen, from, to, iomap);
> > >
> > > After taking another look at the buffer head path, it does look like we
> > > have slightly different behavior here. IIUC, the former reads only the
> > > !uptodate blocks that fall along the from/to boundaries. Here, if say
> > > from = 1, to = PAGE_SIZE and the page is fully !uptodate, it looks like
> > > we'd read the entire page worth of blocks (assuming contiguous 512b
> > > blocks, for example). Intentional? Doesn't seem like a big deal, but
> > > could be worth a followup fix.
> >
> > It wasn't actuall intentional, but I actually think it is the right thing
> > in then end, as it means we'll often do a single read instead of two
> > separate ones.
>
> Ok, but if that's the argument, then shouldn't we not be doing two
> separate I/Os if the middle range of a write happens to be already
> uptodate? Or more for that matter, if the page happens to be sparsely
> uptodate for whatever reason..?
>
> OTOH, I also do wonder a bit whether that may always be the right thing
> if we consider cases like 64k page size arches and whatnot. It seems
> like we could end up consuming more bandwidth for reads than we
> typically have in the past. That said, unless there's a functional
> reason to change this I think it's fine to optimize this path for these
> kinds of corner cases in follow on patches.
>
> Finally, this survived xfstests on a sub-page block size fs but I
> managed to hit an fsx error:
>
> Mapped Read: non-zero data past EOF (0x21a1f) page offset 0xc00 is
> 0xc769
>
> It repeats 100% of the time for me using the attached fsxops file (with
> --replay-ops) on XFS w/ -bsize=1k. It doesn't occur without the final
> patch to enable sub-page block iomap on XFS.
Funny, because I saw the exact same complaint from generic/127 last
night on my development tree that doesn't include hch's patches and was
going to see if I could figure out what's going on.
FWIW it's been happening sporadically for a few weeks now but every time
I've tried to analyze it I (of course) couldn't get it to reproduce. :)
I also ran this series (all of it, including the subpagesize config)
last night and aside from it stumbling over an unrelated locking problem
seemed fine....
--D
> Brian
>
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