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 <jlayton@xxxxxxxxxx>
