Re: [UPDATE PATCH] mm: shmem: don't truncate page if memory failure happens

[Yang Shi]

On Sun, Nov 14, 2021 at 8:54 AM Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Sun, Nov 14, 2021 at 7:28 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> >
> > On Sat, Nov 13, 2021 at 09:32:21PM -0800, Yang Shi wrote:
> > > @@ -2466,7 +2467,18 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> > >                       return -EPERM;
> > >       }
> > >
> > > -     return shmem_getpage(inode, index, pagep, SGP_WRITE);
> > > +     ret = shmem_getpage(inode, index, pagep, SGP_WRITE);
> > > +
> > > +     if (ret)
> > > +             return ret;
> > > +
> > > +     if (*pagep && PageHWPoison(*pagep)) {
> > > +             unlock_page(*pagep);
> > > +             put_page(*pagep);
> > > +             ret = -EIO;
> >
> > You definitely need to add:
> >
> >                 *pagep = NULL;
>
> Thanks, will do it.
>
> >
> > I'm not entirely convinced that you need the conditional on '*pagep'.
> > If we returned 0, there had better be a page at pagep!
>
> For SGP_WRITE, yes, it has a page at pagep if ret is 0, but
> shmem_getpage() could return 0 with NULL page at pagep for SGP_READ.
>
> In the other thread that Linus elaborated why this commit was reverted
> and needed some rework, the discussion about not relying on
> implementation detail for error handling taught me it may be not a
> robust implementation to assume it is never NULL.
>
> We might refactor shmem_getpage() code in the future to make sure when
> it returns success there must be a valid pagep so that we just need to
> care about the return value for error handling.
>
> >
> > I also think this would be clearer if written as:
> >
> >         if (PageHWPoison(*pagep)) {
> >                 unlock_page(*pagep);
> >                 put_page(*pagep);
> >                 *pagep = NULL;
> >                 return -EIO;
> >         }
> >
> >         return 0;

By rethinking the code, I do prefer the above. "if (*pagep &&
PageHWPoison(*pagep))"  does give extra protection from NULL pointer
dereference for the future, but on the opposite side it seems
confusing to *not* have error handling for NULL page. I bet it may
incur more confusion than the protection for future, anyway it can't
be NULL if ret == 0 and !SGP_READ.

shmem_read_mapping_page_gfp() doesn't check page pointer either. IIUC
only SGP_READ case has NULL page pointer checked in shmem code, so I'd
like to follow the convention in this patch. If we think it is not
good, we could refactor the code (for example, guarantee page have
valid page as long as ret value is not error) in a perarate patch.

> >
> > instead of re-using ret.  Sometimes that can make the code flow clearer,
> > but here, I don't think it does.
>
> Sure.
>
> >
> > > @@ -4168,9 +4201,12 @@ struct page *shmem_read_mapping_page_gfp(struct address_space *mapping,
> > >       error = shmem_getpage_gfp(inode, index, &page, SGP_CACHE,
> > >                                 gfp, NULL, NULL, NULL);
> > >       if (error)
> > > -             page = ERR_PTR(error);
> > > -     else
> > > -             unlock_page(page);
> > > +             return ERR_PTR(error);
> > > +
> > > +     unlock_page(page);
> > > +     if (PageHWPoison(page))
> > > +             return ERR_PTR(-EIO);
> >
> > Do we need to put_page() the page in this error case?
>
> Aha, yes. Sorry for missing this. I was fooled by shmem_pin_map() in
> i915 driver which does put page, but I realized it just puts the valid
> pages pinned *before* meeting error page by second look
> .
> >