Re: [PATCH] fs/proc/task_mmu: fix lost maps entries above 2^63

[Lorenzo Stoakes (ARM)]

On Tue, Oct 13, 2026 at 04:40:43PM +0200, David Hildenbrand (Arm) wrote:
> On 10/11/26 18:20, Stian Halseth wrote:
> > When a read of /proc/pid/{maps,smaps,numa_maps} resumes, m_start()
> > rewinds *ppos to priv->last_pos so that the vma which did not fit into
> > the previous buffer is shown again. It only does so for last_addr > 0,
> > meaning to exclude the start of the file and SENTINEL_VMA_GATE, but
> > last_addr is a signed loff_t holding a vm_end. sparc64 maps shared
> > libraries, anonymous memory and the stack above 0x8000000000000000, so
> > the rewind does not happen there and one entry is lost at every read()
> > boundary.
> >
> > Reading the maps file of a process with read sizes from 64 to 8192
> > bytes in steps of 64 loses lines at 126 of the 128 sizes, smaps at 123
> > and numa_maps at 97, and the proc-maps-race selftest fails 4 of its 6
> > tests. Python's sys.remote_exec() reads maps with fgets() and fails
> > intermittently when it misses the libpython data mapping.
> >
> > Test for the two values that are not a vma end instead.
> >
> > Fixes: 5631da56c9a87 ("fs/proc/task_mmu: read proc/pid/maps under per-vma lock")
> > Closes: https://github.com/sparclinux/issues/issues/111
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Stian Halseth <stian@xxxxxx>
> > ---
> >
> > Notes:
> >     Tested on a SPARC T7-1 (sun4v, 7.3-rc5), a T4-1 (7.2.9) and a Sun Fire
> >     V240 (sun4u, 7.3-rc5): no lost lines at any read size afterwards, and
> >     Python's sys.remote_exec() test passes 20/20. On an amd64 KVM guest maps,
> >     smaps, numa_maps and the proc selftests are unchanged.
> >
> >  fs/proc/task_mmu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index 5c54aebe21182..cdce513edcb1d 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -339,7 +339,7 @@ static void *m_start(struct seq_file *m, loff_t *ppos)
> >  	 * Reset current position if last_addr was set before
> >  	 * and it's not a sentinel.

It's interesting that this comment differs from the actual code :)

> >  	 */
> > -	if (last_addr > 0)
> > +	if (last_addr && last_addr != SENTINEL_VMA_GATE)
> >  		*ppos = last_addr = priv->last_pos;
> >  	vma_iter_init(&priv->iter, mm, (unsigned long)last_addr);
> >  	hold_task_mempolicy(priv);
>
> I hope Suren can review as well.

Yes Suren should take a look.

>
> Would it be cleaner to reject any special values, that is, SENTINEL_VMA_END and
> SENTINEL_VMA_GATE?
>
> That would match the comment and the previous code idea: exclude only 0 and
> sentinels.
>
> Could consider a simple addr_is_sentinel() helper.

Yeah that seems sensible.

I guess Stian should respin with something like that?

--
Cheers, Lorenzo