linux-kernel - Re: [PATCH v6 12/20] mm: shmem: allow freezing inode mapping

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Message-ID: <CA+CK2bC7O4B=R7Wb2wZ7QYH2_Ujo-REXVqUX1ukfPJ-XDubtLA@mail.gmail.com>
Date: Mon, 17 Nov 2025 23:13:39 -0500
From: Pasha Tatashin <pasha.tatashin@...een.com>
To: Mike Rapoport <rppt@...nel.org>
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Subject: Re: [PATCH v6 12/20] mm: shmem: allow freezing inode mapping

> > +/* Must be called with inode lock taken exclusive. */
> > +static inline void shmem_i_mapping_freeze(struct inode *inode, bool freeze)
>
> _mapping usually refers to operations on struct address_space.
> It seems that all shmem methods that take inode are just shmem_<operation>,
> so shmem_freeze() looks more appropriate.

Done, renamed to shmem_freeze()

>
> > +{
> > +     if (freeze)
> > +             SHMEM_I(inode)->flags |= SHMEM_F_MAPPING_FROZEN;
> > +     else
> > +             SHMEM_I(inode)->flags &= ~SHMEM_F_MAPPING_FROZEN;
> > +}
> > +
> >  /*
> >   * If fallocate(FALLOC_FL_KEEP_SIZE) has been used, there may be pages
> >   * beyond i_size's notion of EOF, which fallocate has committed to reserving:
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index 1d5036dec08a..05c3db840257 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1292,7 +1292,8 @@ static int shmem_setattr(struct mnt_idmap *idmap,
> >               loff_t newsize = attr->ia_size;
> >
> >               /* protected by i_rwsem */
> > -             if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
> > +             if ((info->flags & SHMEM_F_MAPPING_FROZEN) ||
>
> A corner case: if newsize == oldsize this will be a false positive

Added a fix.

Thanks,
Pasha

>
> > +                 (newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
> >                   (newsize > oldsize && (info->seals & F_SEAL_GROW)))
> >                       return -EPERM;
> >
> > @@ -3289,6 +3290,10 @@ shmem_write_begin(const struct kiocb *iocb, struct address_space *mapping,
> >                       return -EPERM;
> >       }
> >
> > +     if (unlikely((info->flags & SHMEM_F_MAPPING_FROZEN) &&
> > +                  pos + len > inode->i_size))
> > +             return -EPERM;
> > +
> >       ret = shmem_get_folio(inode, index, pos + len, &folio, SGP_WRITE);
> >       if (ret)
> >               return ret;
> > @@ -3662,6 +3667,11 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> >
> >       inode_lock(inode);
> >
> > +     if (info->flags & SHMEM_F_MAPPING_FROZEN) {
> > +             error = -EPERM;
> > +             goto out;
> > +     }
> > +
> >       if (mode & FALLOC_FL_PUNCH_HOLE) {
> >               struct address_space *mapping = file->f_mapping;
> >               loff_t unmap_start = round_up(offset, PAGE_SIZE);
> > --
> > 2.52.0.rc1.455.g30608eb744-goog
> >
>
> --
> Sincerely yours,
> Mike.