Re: [v4 PATCH 5/6] mm: shmem: don't truncate page if memory failure happens

[Yang Shi <shy828301@gmail.com>]

On Mon, Oct 18, 2021 at 10:52 PM Naoya Horiguchi
<naoya.horiguchi@linux.dev> wrote:
>
> On Thu, Oct 14, 2021 at 12:16:14PM -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@gmail.com>
> > ---
> >  mm/memory-failure.c | 10 +++++++++-
> >  mm/shmem.c          | 37 ++++++++++++++++++++++++++++++++++---
> >  mm/userfaultfd.c    |  5 +++++
> >  3 files changed, 48 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index cdf8ccd0865f..f5eab593b2a7 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -57,6 +57,7 @@
> >  #include <linux/ratelimit.h>
> >  #include <linux/page-isolation.h>
> >  #include <linux/pagewalk.h>
> > +#include <linux/shmem_fs.h>
> >  #include "internal.h"
> >  #include "ras/ras_event.h"
> >
> > @@ -866,6 +867,7 @@ static int me_pagecache_clean(struct page_state *ps, struct page *p)
> >  {
> >       int ret;
> >       struct address_space *mapping;
> > +     bool extra_pins;
> >
> >       delete_from_lru_cache(p);
> >
> > @@ -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 is expected to have an extra refcount after error-handling.
> > +      */
> > +     extra_pins = shmem_mapping(mapping);
> > +
> >       /*
> >        * Truncation is a bit tricky. Enable it per file system for now.
> >        *
> > @@ -903,7 +911,7 @@ static int me_pagecache_clean(struct page_state *ps, struct page *p)
> >  out:
> >       unlock_page(p);
> >
> > -     if (has_extra_refcount(ps, p, false))
> > +     if (has_extra_refcount(ps, p, extra_pins))
> >               ret = MF_FAILED;
> >
> >       return ret;
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index b5860f4a2738..69eaf65409e6 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -2456,6 +2456,7 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> >       struct inode *inode = mapping->host;
> >       struct shmem_inode_info *info = SHMEM_I(inode);
> >       pgoff_t index = pos >> PAGE_SHIFT;
> > +     int ret = 0;
> >
> >       /* i_rwsem is held by caller */
> >       if (unlikely(info->seals & (F_SEAL_GROW |
> > @@ -2466,7 +2467,15 @@ 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 && PageHWPoison(*pagep)) {
>
> shmem_getpage() could return with pagep == NULL, so you need check ret first
> to avoid NULL pointer dereference.

Realy? IIUC pagep can't be NULL. It is a pointer's pointer passed in
by the caller, for example, generic_perform_write(). Of course,
"*pagep" could be NULL.

>
> > +             unlock_page(*pagep);
> > +             put_page(*pagep);
> > +             ret = -EIO;
> > +     }
> > +
> > +     return ret;
> >  }
> >
> >  static int
> > @@ -2555,6 +2564,11 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> >                       unlock_page(page);
> >               }
> >
> > +             if (page && PageHWPoison(page)) {
> > +                     error = -EIO;
>
> Is it cleaner to add PageHWPoison() check in the existing "if (page)" block
> just above?  Then, you don't have to check "page != NULL" twice.
>
> @@ -2562,7 +2562,11 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
>                         if (sgp == SGP_CACHE)
>                                 set_page_dirty(page);
>                         unlock_page(page);
>
> +                       if (PageHWPoison(page)) {
> +                               error = -EIO;
> +                               break;
> +                       }

Yeah, it looks better indeed.

>                 }
>
>                 /*
>
>
> Thanks,
> Naoya Horiguchi