Re: [RFC PATCH v2] implement orangefs_readahead

[Mike Marshall <hubcap@omnibond.com>]

>> What happens if bytes_remaining < PAGE_SIZE?

I think on a call where that occurs, new_len won't get set
and readahead_expand won't get called. I don't see how that's
not correct, but I question me more than I question you :-) ...

>> what happens if bytes_remaining % PAGE_SIZE != 0

I think bytes_remaining % PAGE_SIZE worth of bytes won't get read on
that call, but that the readahead callout keeps getting called until all
the bytes are read? After you asked this, I thought about adding 1 to
new_len in such cases, and did some tests that way, it seems to me like it
works out as is.

>> I wonder if you should use iov_length(&iter)

iov_length has two arguments. The first one would maybe be iter.iov and
the second one would be... ?

>> should you cache inode->i_size lest it change under you due to truncate

That seems important, but I can't return an error from the void
readahead callout. Would I react by somehow returning the pages
back to their original condition, or ?

Anywho... I see that you've force pushed a new netfs... I think you
have it applied to a linus-tree-of-the-day on top of 5.12-rc4?
I have taken these patches from
git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git (netfs-lib)

0001-iov_iter-Add-ITER_XARRAY.patch
0002-mm-Add-set-end-wait-functions-for-PG_private_2.patch
0003-mm-filemap-Pass-the-file_ra_state-in-the-ractl.patch
0004-fs-Document-file_ra_state.patch
0005-mm-readahead-Handle-ractl-nr_pages-being-modified.patch
0006-mm-Implement-readahead_control-pageset-expansion.patch
0007-netfs-Make-a-netfs-helper-module.patch
0008-netfs-Documentation-for-helper-library.patch
0009-netfs-mm-Move-PG_fscache-helper-funcs-to-linux-netfs.patch
0010-netfs-mm-Add-set-end-wait_on_page_fscache-aliases.patch
0011-netfs-Provide-readahead-and-readpage-netfs-helpers.patch
0012-netfs-Add-tracepoints.patch
0013-netfs-Gather-stats.patch
0014-netfs-Add-write_begin-helper.patch
0015-netfs-Define-an-interface-to-talk-to-a-cache.patch
0016-netfs-Add-a-tracepoint-to-log-failures-that-would-be.patch
0017-fscache-cachefiles-Add-alternate-API-to-use-kiocb-fo.patch

... and added them on top of Linux 5.12-rc8 and added my
readahead patch to that.

Now I fail one extra xfstest, I fail generic/075, generic/112,
generic/127, generic/263 and generic/438. I haven't found an
obvious (to me) problem with my patch and I can't claim to understand
everything that is going on in the patches I have of yours...

I'll keep looking...

-Mike

On Fri, Apr 16, 2021 at 11:41 AM David Howells <dhowells@redhat.com> wrote:
>
> In orangefs_readahead():
>
>         loff_t bytes_remaining = inode->i_size - readahead_pos(rac);
>         loff_t pages_remaining = bytes_remaining / PAGE_SIZE;
>
> What happens if bytes_remaining < PAGE_SIZE?  Or even what happens if
> bytes_remaining % PAGE_SIZE != 0?
>
>         if ((ret = wait_for_direct_io(ORANGEFS_IO_READ, inode,
>                         &offset, &iter, readahead_length(rac),
>                         inode->i_size, NULL, NULL, file)) < 0)
>
> I wonder if you should use iov_length(&iter) rather than
> readahead_length(rac).  They *should* be the same.
>
> Also, should you cache inode->i_size lest it change under you due to truncate?
>
> David
>