Re: [PATCH v1] exfat: mark straddling folio RO for 4K block size

[Yuezhang.Mo@xxxxxxxx]

> >   #include "exfat_raw.h"
> >   #include "exfat_fs.h"
> > @@ -91,6 +92,20 @@ static int exfat_cont_expand(struct inode *inode, loff_t size)
> >        */
> >       pagecache_isize_extended(inode, oldsize, inode->i_size);
> >
> > +     if (i_blocksize(inode) >= PAGE_SIZE && (oldsize & (PAGE_SIZE - 1))) {
> > +             struct folio *folio;
> > +
> > +             folio = filemap_lock_folio(inode->i_mapping,
> > +                             oldsize >> PAGE_SHIFT);
> > +             if (!IS_ERR(folio)) {
> > +                     if (folio_mkclean(folio))
> > +                             folio_mark_dirty(folio);
> > +
> > +                     folio_unlock(folio);
> > +                     folio_put(folio);
> > +             }
> > +     }
>
> I think fixing this issue specifically for the block size = 4K case may
> not be thorough enough, because pagecache_isize_extended() may also fail
> to work when the block size is smaller than 4K.

What condition might pagecache_isize_extended() fail to work when the block
size is smaller than 4K?

>
> We should set the folio containing valid_size to RO whenever valid_size
> != i_size and valid_size is not page-aligned, regardless of the block size.

We should set the folio containing oldsize to RO, rather than valid_size.
This is because, when the folio containing oldsize was first set to RW, oldsize
constrained valid_size, after the file size is subsequently expanded, that 
constraint changes, necessitating a re-setting of valid_size.