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Sun, 14 Sep 2025 18:47:38 -0700 (PDT) Date: Mon, 15 Sep 2025 01:47:38 +0000 From: Wei Yang To: Lance Yang Cc: Wei Yang , linux-mm@kvack.org, chengming.zhou@linux.dev, ryan.roberts@arm.com, baohua@kernel.org, xu.xin16@zte.com.cn, dev.jain@arm.com, npache@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, akpm@linux-foundation.org, ziy@nvidia.com, david@redhat.com, baolin.wang@linux.alibaba.com Subject: Re: [PATCH] mm/mm_slot: make sure slot is the first element of its wrapper structure Message-ID: <20250915014738.slkw4nwihb4aavas@master> Reply-To: Wei Yang References: <20250914000026.17986-1-richard.weiyang@gmail.com> <3a1adf4c-9931-4cdb-a024-37c856b1702b@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3a1adf4c-9931-4cdb-a024-37c856b1702b@linux.dev> User-Agent: NeoMutt/20170113 (1.7.2) X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: F09E8120004 X-Stat-Signature: e8xtbwnnnewesx816o6ar9khyoiu88ot X-Rspam-User: X-HE-Tag: 1757900861-929087 X-HE-Meta: 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 sF2cW7xS 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 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 Sun, Sep 14, 2025 at 01:05:48PM +0800, Lance Yang wrote: >Hi Wei, > >On 2025/9/14 08:00, Wei Yang wrote: >> When using mm_slot in ksm/khugepaged, there is code snip like: >> >> slot = mm_slot_lookup(mm_slots_hash, mm); >> mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot); >> if (mm_slot && ..) { >> } >> >> This is only valid when mm_slot is the first element of its wrapper >> structure, otherwise a NULL slot would converted to a mm_slot with >> negative value. And current code thinks it is valid and continue. >> >> Current code works since mm_slot is the first element, but make sure it >> won't be disturbed. > >Good catch! That's indeed quite brittle ;) > >Just one nit below. > >> >> Signed-off-by: Wei Yang >> --- >> mm/khugepaged.c | 5 ++++- >> mm/ksm.c | 5 ++++- >> 2 files changed, 8 insertions(+), 2 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index af5f5c80fe4e..668e74ad33b7 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -108,7 +108,7 @@ struct collapse_control { >> * @slot: hash lookup from mm to mm_slot >> */ >> struct khugepaged_mm_slot { >> - struct mm_slot slot; >> + struct mm_slot slot; /* keep it the first element */ >> }; >> /** >> @@ -2382,6 +2382,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result, >> struct vm_area_struct *vma; >> int progress = 0; >> + BUILD_BUG_ON_MSG(mm_slot_entry(NULL, struct khugepaged_mm_slot, slot), >> + "slot should be the first element"); >> + > >I wonder if this BUILD_BUG_ON_MSG() in khugepaged_scan_mm_slot() would >be better placed in khugepaged_init(), like you did in ksm_init()? khugepaged_mm_slot is defined in khugepaged.c, maybe we don't want to export it. > >Cheers, >Lance > >> VM_BUG_ON(!pages); >> lockdep_assert_held(&khugepaged_mm_lock); >> *result = SCAN_FAIL; >> diff --git a/mm/ksm.c b/mm/ksm.c >> index 2ef29802a49b..0d486dbdf7d3 100644 >> --- a/mm/ksm.c >> +++ b/mm/ksm.c >> @@ -124,7 +124,7 @@ typedef u8 rmap_age_t; >> * @rmap_list: head for this mm_slot's singly-linked list of rmap_items >> */ >> struct ksm_mm_slot { >> - struct mm_slot slot; >> + struct mm_slot slot; /* keep it the first element */ >> struct ksm_rmap_item *rmap_list; >> }; >> @@ -3842,6 +3842,9 @@ static int __init ksm_init(void) >> struct task_struct *ksm_thread; >> int err; >> + BUILD_BUG_ON_MSG(mm_slot_entry(NULL, struct ksm_mm_slot, slot), >> + "slot should be the first element"); >> + >> /* The correct value depends on page size and endianness */ >> zero_checksum = calc_checksum(ZERO_PAGE(0)); >> /* Default to false for backwards compatibility */ -- Wei Yang Help you, Help me