linux-kernel - Re: [PATCH] smaps: Fix the abnormal memory statistics obtained through /proc/pid/smaps

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Date:   Thu, 27 Jul 2023 13:37:06 +0200
From:   David Hildenbrand <david@...hat.com>
To:     liubo <liubo254@...wei.com>
Cc:     akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, hughd@...gle.com, peterx@...hat.com,
        willy@...radead.org
Subject: Re: [PATCH] smaps: Fix the abnormal memory statistics obtained
 through /proc/pid/smaps

On Wed, Jul 26, 2023 at 9:40 AM liubo <liubo254@...wei.com> 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.

> Signed-off-by: liubo <liubo254@...wei.com>
> 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().