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
