Hi,
On 2020-03-09 13:50:59 -0400, Jeff Layton wrote:
> The PostgreSQL devs asked a while back for some way to tell whether
> there have been any writeback errors on a superblock w/o having to do
> any sort of flush -- just "have there been any so far".
Indeed.
> I sent a patch a few weeks ago to make syncfs() return errors when there
> have been writeback errors on the superblock. It's not merged yet, but
> once we have something like that in place, we could expose info from the
> errseq_t to userland using this interface.
I'm still a bit worried about the details of errseq_t being exposed to
userland. Partially because it seems to restrict further evolution of
errseq_t, and partially because it will likely up with userland trying
to understand it (it's e.g. just too attractive to report a count of
errors etc).
Is there a reason to not instead report a 64bit counter instead of the
cookie? In contrast to the struct file case we'd only have the space
overhead once per superblock, rather than once per #files * #fd. And it
seems that the maintenance of that counter could be done without
widespread changes, e.g. instead/in addition to your change:
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index ccb14b6a16b5..897439475315 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -51,7 +51,10 @@ static inline void mapping_set_error(struct address_space *mapping, int error)
> return;
>
> /* Record in wb_err for checkers using errseq_t based tracking */
> - filemap_set_wb_err(mapping, error);
> + __filemap_set_wb_err(mapping, error);
> +
> + /* Record it in superblock */
> + errseq_set(&mapping->host->i_sb->s_wb_err, error);
>
> /* Record it in flags for now, for legacy callers */
> if (error == -ENOSPC)
Btw, seems like mapping_set_error() should have a non-inline cold path?
Greetings,
Andres Freund
On Mon, 2020-03-09 at 12:22 -0700, Andres Freund wrote:
> Hi,
>
> On 2020-03-09 13:50:59 -0400, Jeff Layton wrote:
> > The PostgreSQL devs asked a while back for some way to tell whether
> > there have been any writeback errors on a superblock w/o having to do
> > any sort of flush -- just "have there been any so far".
>
> Indeed.
>
>
> > I sent a patch a few weeks ago to make syncfs() return errors when there
> > have been writeback errors on the superblock. It's not merged yet, but
> > once we have something like that in place, we could expose info from the
> > errseq_t to userland using this interface.
>
> I'm still a bit worried about the details of errseq_t being exposed to
> userland. Partially because it seems to restrict further evolution of
> errseq_t, and partially because it will likely up with userland trying
> to understand it (it's e.g. just too attractive to report a count of
> errors etc).
Trying to interpret the counter field won't really tell you anything.
The counter is not incremented unless someone has queried the value
since it was last checked. A single increment could represent a single
writeback error or 10000 identical ones.
There _is_ a flag that tells you whether someone has queried it, but
that gets masked off before copying the cookie to userland.
> Is there a reason to not instead report a 64bit counter instead of the
> cookie? In contrast to the struct file case we'd only have the space
> overhead once per superblock, rather than once per #files * #fd. And it
> seems that the maintenance of that counter could be done without
> widespread changes, e.g. instead/in addition to your change:
>
What problem would moving to a 64-bit counter solve? I get the concern
about people trying to get a counter out of the cookie field, but giving
people an explicit 64-bit counter seems even more open to
misinterpretation.
All that said, is an opaque cookie still something you'd find useful?
> > diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> > index ccb14b6a16b5..897439475315 100644
> > --- a/include/linux/pagemap.h
> > +++ b/include/linux/pagemap.h
> > @@ -51,7 +51,10 @@ static inline void mapping_set_error(struct address_space *mapping, int error)
> > return;
> >
> > /* Record in wb_err for checkers using errseq_t based tracking */
> > - filemap_set_wb_err(mapping, error);
> > + __filemap_set_wb_err(mapping, error);
> > +
> > + /* Record it in superblock */
> > + errseq_set(&mapping->host->i_sb->s_wb_err, error);
> >
> > /* Record it in flags for now, for legacy callers */
> > if (error == -ENOSPC)
>
> Btw, seems like mapping_set_error() should have a non-inline cold path?
Good point. I'll do that in the next iteration.
--
Jeff Layton <[email protected]>