Re: [v3 PATCH 4/5] mm: shmem: don't truncate page if memory failure happens

[Yang Shi]

On Fri, Oct 1, 2021 at 12:05 AM Naoya Horiguchi
<naoya.horiguchi@xxxxxxxxx> wrote:
>
> On Thu, Sep 30, 2021 at 02:53:10PM -0700, Yang Shi wrote:
> > The current behavior of memory failure is to truncate the page cache
> > regardless of dirty or clean.  If the page is dirty the later access
> > will get the obsolete data from disk without any notification to the
> > users.  This may cause silent data loss.  It is even worse for shmem
> > since shmem is in-memory filesystem, truncating page cache means
> > discarding data blocks.  The later read would return all zero.
> >
> > The right approach is to keep the corrupted page in page cache, any
> > later access would return error for syscalls or SIGBUS for page fault,
> > until the file is truncated, hole punched or removed.  The regular
> > storage backed filesystems would be more complicated so this patch
> > is focused on shmem.  This also unblock the support for soft
> > offlining shmem THP.
> >
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > ---
> ...
> > @@ -894,6 +896,12 @@ static int me_pagecache_clean(struct page_state *ps, struct page *p)
> >               goto out;
> >       }
> >
> > +     /*
> > +      * The shmem page is kept in page cache instead of truncating
> > +      * so need decrement the refcount from page cache.
> > +      */
>
> This comment seems to me confusing because no refcount is decremented here.
> What the variable dec tries to do is to give the expected value of the
> refcount of the error page after successfull erorr handling, which differs
> according to the page state before error handling, so dec adjusts it.
>
> How about the below?
>
> +       /*
> +        * The shmem page is kept in page cache instead of truncating
> +        * so is expected to have an extra refcount after error-handling.
> +        */

Thanks for the suggestion, yes, it seems better.

>
> > +     dec = shmem_mapping(mapping);
> > +
> >       /*
> >        * Truncation is a bit tricky. Enable it per file system for now.
> >        *
> ...
> > @@ -2466,7 +2467,17 @@ 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 (*pagep) {
> > +             if (PageHWPoison(*pagep)) {
>
> Unless you plan to add some code in the near future, how about merging
> these two if sentences?
>
>         if (*pagep && PageHWPoison(*pagep)) {

Sure.

>
> Thanks,
> Naoya Horiguchi
>
> > +                     unlock_page(*pagep);
> > +                     put_page(*pagep);
> > +                     ret = -EIO;
> > +             }
> > +     }
> > +
> > +     return ret;
> >  }
> >
> >  static int