On Thu, Jul 27, 2023 at 01:37:06PM +0200, David Hildenbrand wrote:
> On Wed, Jul 26, 2023 at 9:40 AM liubo <liubo254@xxxxxxxxxx> wrote:
> >
> > In commit 474098edac26 ("mm/gup: replace FOLL_NUMA by
> > gup_can_follow_protnone()"), FOLL_NUMA was removed and replaced by
> > the gup_can_follow_protnone interface.
> >
> > However, for the case where the user-mode process uses transparent
> > huge pages, when analyzing the memory usage through
> > /proc/pid/smaps_rollup, the obtained memory usage is not consistent
> > with the RSS in /proc/pid/status.
> >
> > Related examples are as follows:
> > cat /proc/15427/status
> > VmRSS: 20973024 kB
> > RssAnon: 20971616 kB
> > RssFile: 1408 kB
> > RssShmem: 0 kB
> >
> > cat /proc/15427/smaps_rollup
> > 00400000-7ffcc372d000 ---p 00000000 00:00 0 [rollup]
> > Rss: 14419432 kB
> > Pss: 14418079 kB
> > Pss_Dirty: 14418016 kB
> > Pss_Anon: 14418016 kB
> > Pss_File: 63 kB
> > Pss_Shmem: 0 kB
> > Anonymous: 14418016 kB
> > LazyFree: 0 kB
> > AnonHugePages: 14417920 kB
> >
> > The root cause is that the traversal In the page table, the number of
> > pages obtained by smaps_pmd_entry does not include the pages
> > corresponding to PROTNONE,resulting in a different situation.
> >
>
> Thanks for reporting and debugging!
>
> > Therefore, when obtaining pages through the follow_trans_huge_pmd
> > interface, add the FOLL_FORCE flag to count the pages corresponding to
> > PROTNONE to solve the above problem.
> >
>
> We really want to avoid the usage of FOLL_FORCE, and ideally limit it
> to ptrace only.
Fundamentally when removing FOLL_NUMA we did already assumed !FORCE is
FOLL_NUMA. It means to me after the removal it's not possible to say in a
gup walker that "it's not FORCEd, but I don't want to trigger NUMA but just
get the page".
Is that what we want? Shall we document that in FOLL_FORCE if we intended
to enforce numa balancing as long as !FORCE?
>
> > Signed-off-by: liubo <liubo254@xxxxxxxxxx>
> > Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
> > ---
> > fs/proc/task_mmu.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > index c1e6531cb02a..ed08f9b869e2 100644
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -571,8 +571,10 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> > bool migration = false;
> >
> > if (pmd_present(*pmd)) {
> > - /* FOLL_DUMP will return -EFAULT on huge zero page */
> > - page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> > + /* FOLL_DUMP will return -EFAULT on huge zero page
> > + * FOLL_FORCE follow a PROT_NONE mapped page
> > + */
> > + page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP | FOLL_FORCE);
> > } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
> > swp_entry_t entry = pmd_to_swp_entry(*pmd);
>
> Might do as an easy fix. But we really should get rid of that
> absolutely disgusting usage of follow_trans_huge_pmd().
>
> We don't need 99% of what follow_trans_huge_pmd() does here.
>
> Would the following also fix your issue?
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 507cd4e59d07..fc744964816e 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -587,8 +587,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> bool migration = false;
>
> if (pmd_present(*pmd)) {
> - /* FOLL_DUMP will return -EFAULT on huge zero page */
> - page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
> + page = vm_normal_page_pmd(vma, addr, *pmd);
> } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
> swp_entry_t entry = pmd_to_swp_entry(*pmd);
>
> It also skips the shared zeropage and pmd_devmap(),
>
> Otherwise, a simple pmd_page(*pmd) + is_huge_zero_pmd(*pmd) check will do, but I
> suspect vm_normal_page_pmd() might be what we actually want to have here.
>
> Because smaps_pte_entry() properly checks for vm_normal_page().
There're indeed some very trivial detail in vm_normal_page_pmd() that's
different, but maybe not so relevant. E.g.,
if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
return -ENOMEM;
if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
return -EREMOTEIO;
I'm not sure whether the p2pdma page would matter in any form here. E.g.,
whether it can be mapped privately.
--
Peter Xu
