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b=EnISaw+MJcidIWBxl3b2zyOxLiLqneya2LRIly1pSz+adbT+hIsMBszDoq/1XxQzT EOwmS08G5yDnzw3r7FRUZbwaLNcO/rA2YGJYEh6jGR7CihhTNJ3W5qnRq9k8bhEE0f w+Zj/ndDQj8tzdfVTWrkrIpIj6lgzt8m3oN54PgePS0YgN3ZLCtqEEYtP1dVZmviF+ GXwlHxwRsG6SIi1jjZ7Hb+inHCPLpl8RCg4taWSRR2ZmA+H6GBWHVBVnnPKxhFs8z3 3pc/+pqwE7BOkwJMB53XTwf6BAlt0meA22RFzZYte+rzecCVAlxWKeH6lZ0/orHghG v+pgANAGokrGQ== Message-ID: <9924fc6c-e3ad-422a-ad60-756efccba0aa@kernel.org> Date: Wed, 14 Jan 2026 12:40:50 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-new v4 3/6] mm: khugepaged: just skip when the memory has been collapsed To: Vernon Yang , akpm@linux-foundation.org Cc: lorenzo.stoakes@oracle.com, ziy@nvidia.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang References: <20260111121909.8410-1-yanglincheng@kylinos.cn> <20260111121909.8410-4-yanglincheng@kylinos.cn> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 11BE6A0003 X-Rspam-User: X-Stat-Signature: gcxyd6iayb57b54wm8b9ygmarpmruuqd X-HE-Tag: 1768390854-364739 X-HE-Meta: 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 rgv2wNC2 0ClFU7rYvZG4L6+WGK108gn13jL592OHvvs48ca3plnscbJPINyPzqJHX72T05RuPTglwGgFBf6Jz0tL49AOS4BYm4wEg46SLiWIEHN8nt6CUFisP+SXHwfLVNHQ2sevhp3UrXvOxUQ+xgkHhzHArpABenhnGGdiI+4yYFhgJpY4CJtHk+qwVB4JfapybcGvAfU3eiQBn5nt4ht8yL/eNx1noKGrVtFh/6Nw6 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1/11/26 13:19, Vernon Yang wrote: > The following data is traced by bpftrace on a desktop system. After > the system has been left idle for 10 minutes upon booting, a lot of > SCAN_PMD_MAPPED or SCAN_NO_PTE_TABLE are observed during a full scan > by khugepaged. > > @scan_pmd_status[1]: 1 ## SCAN_SUCCEED > @scan_pmd_status[6]: 2 ## SCAN_EXCEED_SHARED_PTE > @scan_pmd_status[3]: 142 ## SCAN_PMD_MAPPED > @scan_pmd_status[2]: 178 ## SCAN_NO_PTE_TABLE > total progress size: 674 MB > Total time : 419 seconds ## include khugepaged_scan_sleep_millisecs > > The khugepaged_scan list save all task that support collapse into hugepage, > as long as the task is not destroyed, khugepaged will not remove it from > the khugepaged_scan list. This exist a phenomenon where task has already > collapsed all memory regions into hugepage, but khugepaged continues to > scan it, which wastes CPU time and invalid, and due to > khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for > scanning a large number of invalid task, so scanning really valid task > is later. > > After applying this patch, when the memory is either SCAN_PMD_MAPPED or > SCAN_NO_PTE_TABLE, just skip it, as follow: > > @scan_pmd_status[6]: 2 > @scan_pmd_status[3]: 147 > @scan_pmd_status[2]: 173 > total progress size: 45 MB > Total time : 20 seconds > > Signed-off-by: Vernon Yang > --- > mm/khugepaged.c | 17 +++++++++++------ > 1 file changed, 11 insertions(+), 6 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 5c6015ac7b5e..6df2857d94c6 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -68,7 +68,10 @@ enum scan_result { > static struct task_struct *khugepaged_thread __read_mostly; > static DEFINE_MUTEX(khugepaged_mutex); > > -/* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */ > +/* > + * default scan 8*HPAGE_PMD_NR ptes, pmd_mapped, no_pte_table or vmas > + * every 10 second. > + */ > static unsigned int khugepaged_pages_to_scan __read_mostly; > static unsigned int khugepaged_pages_collapsed; > static unsigned int khugepaged_full_scans; > @@ -1267,7 +1270,7 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, > result = find_pmd_or_thp_or_none(mm, start_addr, &pmd); > if (result != SCAN_SUCCEED) { > if (cur_progress) > - *cur_progress = HPAGE_PMD_NR; > + *cur_progress = 1; > goto out; > } > > @@ -1276,7 +1279,7 @@ static enum scan_result hpage_collapse_scan_pmd(struct mm_struct *mm, > pte = pte_offset_map_lock(mm, pmd, start_addr, &ptl); > if (!pte) { > if (cur_progress) > - *cur_progress = HPAGE_PMD_NR; > + *cur_progress = 1; > result = SCAN_NO_PTE_TABLE; > goto out; > } The above checks are clear. > @@ -2347,9 +2350,6 @@ static enum scan_result hpage_collapse_scan_file(struct mm_struct *mm, unsigned > continue; > } > > - if (cur_progress) > - *cur_progress += folio_nr_pages(folio); > - This is a all a bit hairy. Assume we found a single 4k folio in the xarray, but then collapse a 2M THP. Is the progress really "1" ? What about shmem swap entries (xa_is_value)? So I think the whole file path needs more thought -- Cheers David