by Naoya Horiguchi

[permalink] [raw]

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

On Tue, Oct 19, 2021 at 10:29:51AM -0700, Yang Shi wrote:
> On Mon, Oct 18, 2021 at 10:52 PM Naoya Horiguchi
> <[email protected]> wrote:
> >
> > On Thu, Oct 14, 2021 at 12:16:14PM -0700, Yang Shi wrote:
...
> > > @@ -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.

Oh, I simply missed this. You're right. Please ignore my comment on this.

- Naoya Horiguchi

2021-10-20 18:35:10

by Yang Shi

[permalink] [raw]

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

On Mon, Oct 18, 2021 at 10:52 PM Naoya Horiguchi
<[email protected]> 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 <[email protected]>
> > ---
> > 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.
>
> > + 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;

Further looking shows I missed a "put_page" in the first place. Will
fix in the next version too.

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