Re: [LSF/MM TOPIC] do we really need PG_error at all?

[NeilBrown]

On Sun, Feb 26 2017, James Bottomley wrote:

> On Mon, 2017-02-27 at 08:03 +1100, NeilBrown wrote:
>> On Sun, Feb 26 2017, James Bottomley wrote:
>> 
>> > [added linux-scsi and linux-block because this is part of our error
>> > handling as well]
>> > On Sun, 2017-02-26 at 09:42 -0500, Jeff Layton wrote:
>> > > Proposing this as a LSF/MM TOPIC, but it may turn out to be me 
>> > > just not understanding the semantics here.
>> > > 
>> > > As I was looking into -ENOSPC handling in cephfs, I noticed that
>> > > PG_error is only ever tested in one place [1] 
>> > > __filemap_fdatawait_range, which does this:
>> > > 
>> > > 	if (TestClearPageError(page))
>> > > 		ret = -EIO;
>> > > 
>> > > This error code will override any AS_* error that was set in the
>> > > mapping. Which makes me wonder...why don't we just set this error 
>> > > in the mapping and not bother with a per-page flag? Could we
>> > > potentially free up a page flag by eliminating this?
>> > 
>> > Note that currently the AS_* codes are only set for write errors 
>> > not for reads and we have no mapping error handling at all for swap
>> > pages, but I'm sure this is fixable.
>> 
>> How is a read error different from a failure to set PG_uptodate?
>> Does PG_error suppress retries?
>
> We don't do any retries in the code above the block layer (or at least
> we shouldn't).

I was wondering about what would/should happen if a read request was
re-issued for some reason.  Should the error flag on the page cause an
immediate failure, or should it try again.
If read-ahead sees a read-error on some future page, is it necessary to
record the error so subsequent read-aheads don't notice the page is
missing and repeatedly try to re-load it?
When the application eventually gets to the faulty page, should a read
be tried then, or is the read-ahead failure permanent?



>
>> > 
>> > From the I/O layer point of view we take great pains to try to 
>> > pinpoint the error exactly to the sector.  We reflect this up by 
>> > setting the PG_error flag on the page where the error occurred.  If 
>> > we only set the error on the mapping, we lose that granularity, 
>> > because the mapping is mostly at the file level (or VMA level for
>> > anon pages).
>> 
>> Are you saying that the IO layer finds the page in the bi_io_vec and
>> explicitly sets PG_error,
>
> I didn't say anything about the mechanism.  I think the function you're
> looking for is fs/mpage.c:mpage_end_io().  layers below block indicate
> the position in the request.  Block maps the position to bio and the
> bio completion maps to page.  So the actual granularity seen in the
> upper layer depends on how the page to bio mapping is done.

If the block layer is just returning the status at a per-bio level (which
makes perfect sense), then this has nothing directly to do with the
PG_error flag.

The page cache needs to do something with bi_error, but it isn't
immediately clear that it needs to set PG_error.

>
>>  rather than just passing an error indication to bi_end_io ??  That
>> would seem to be wrong as the page may not be in the page cache.
>
> Usually pages in the mpage_end_io path are pinned, I think.
>
>>  So I guess I misunderstand you.
>> 
>> > 
>> > So I think the question for filesystem people from us would be do 
>> > you care about this accuracy?  If it's OK just to know an error
>> > occurred somewhere in this file, then perhaps we don't need it.
>> 
>> I had always assumed that a bio would either succeed or fail, and 
>> that no finer granularity could be available.
>
> It does ... but a bio can be as small as a single page.
>
>> I think the question here is: Do filesystems need the pagecache to
>> record which pages have seen an IO error?
>
> It's not just filesystems.  The partition code uses PageError() ... the
> metadata code might as well (those are things with no mapping).  I'm
> not saying we can't remove PG_error; I am saying it's not going to be
> quite as simple as using the AS_ flags.

The partition code could use PageUptodate().
mpage_end_io() calls page_endio() on each page, and on read error that
calls:

			ClearPageUptodate(page);
			SetPageError(page);

are both of these necessary?

fs/buffer.c can use several bios to read a single page.
If any one returns an error, PG_error is set.  When all of them have
completed, if PG_error is clear, PG_uptodate is then set.
This is an opportunistic use of PG_error, rather than an essential use.
It could be "fixed", and would need to be fixed if we were to deprecate
use of PG_error for read errors.
There are probably other usages like this.

Thanks,
NeilBrown


>
> James
>
>> I think that for write errors, there is no value in recording
>> block-oriented error status - only file-oriented status.
>> For read errors, it might if help to avoid indefinite read retries, 
>> but I don't know the code well enough to be sure if this is an issue.
>> 
>> NeilBrown
>> 
>> 
>> > 
>> > James
>> > 
>> > > The main argument I could see for keeping it is that removing it 
>> > > might subtly change the behavior of sync_file_range if you have 
>> > > tasks syncing different ranges in a file concurrently. I'm not 
>> > > sure if that would break any guarantees though.
>> > > 
>> > > Even if we do need it, I think we might need some cleanup here 
>> > > anyway. A lot of readpage operations end up setting that flag 
>> > > when they hit an error. Isn't it wrong to return an error on 
>> > > fsync, just because we had a read error somewhere in the file in 
>> > > a range that was never dirtied?
>> > > 
>> > > --
>> > > [1]: there is another place in f2fs, but it's more or less 
>> > > equivalent to the call site in __filemap_fdatawait_range.
>> > >

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