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a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778483851; bh=LkpLtBg7oCb43ClpBPUq4iq9i0Tn++YKd0G4AX2wqqg=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=pRtc6WveBLKgmpEpXQlwQ+gEzXNOv+ajONlpwi+nkmOlVJUoC5ECxOupiLcnpWqhs n3iIJbuMtuWIimGSuvnO4rLTxn8AgPTAb3n1LpR0/5ROvcQECPylEEb0dNjvidOfwi pfzNs0XhViQaQHvYKMKE4mz+M+eIY8ngqM470wieRlTTwDi0nYYPWwn7IWAFRcmfMk u/Bn6EQGYr6KWvcmwIW9ulyhhJ+GhtGO3S9im9/+UYILdDYvK/UHSJezRzcQkE8LDm eaOwj62qOJwSvjyDo4EwFmIXRuO/b6nosl/vSIS+PbrMPIpdKwsi1hL3P7PpksKosJ TGuQCiOyz7ccA== Message-ID: Date: Mon, 11 May 2026 09:17:27 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/khugepaged: avoid underflow in madvise_collapse for sub-PMD MADV_COLLAPSE To: Wandun Chen , linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: akpm@linux-foundation.org, ljs@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, lance.yang@linux.dev References: <20260511065701.799006-1-chenwandun@lixiang.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: t8po55ku6no9bmrr5pj9qbnsu1xchpfr X-Rspam-User: X-Rspamd-Queue-Id: 77ED11A0011 X-Rspamd-Server: rspam07 X-HE-Tag: 1778483852-980912 X-HE-Meta: U2FsdGVkX1+fPwFUF9ITtIy4VzCVuSYK0mXE1q9XadLiKmn9Tn5TaCxD7yx5qTaRFiXSCd2x44qmURsklvgwMBneBKZqCMVB1x7H1snTX1HNHdIyjeydSevWf8ikSexkwfNJuqbUaSjbc3uV1aXn/mgQcnIndVlNihhg18Bp9a3EGpjLjGvg5JfemU0TyTPH5Q7uB6tE6LAICH0+yhWjr70y/NurtSQB7HSAAtfzVPUY2B5s1Gnxt1zN91pyGMfscRvtc1yDHA7yIrmM7UF/NUlYTpvYE8phfYbzPEW/VZSc/QCUU5W6rmpZOlpPUzUy65zbQUupO6J3RBZxm5r0YpdeXa7TEgMvRRiwOrWQep/+GpJmg7AHfJiGbKI1J+JNoql6oef/1FTKZT5yk1tURNlrLwKgd5m30kKxQq9GajkQiFpq/am+TJbk8wcf8DLE/ZZEGiXKO9c6WrtwFJGGelF1IpCdYPuraMkclX/I5OO+c0xQ5PNHEvVvEZH1GnxEstdA+DzN20rbbYMzvx06V4NGE30XRzpS+lUo2YMjZNsI7MjkiPff+4g0SPhaaav9TqlAfZCmAAexBE9eoFWpW03UTLvD23x5byzsuWguk8/5HNRNjpdvlfaawIxpnLxqTVdQX5Hchx4znYWWD7gZTuhjfLVd7sCuHZxBjZsc21LZl3DYZE3v4oos5nEAAElPxt4AkIVpzM0VA8uZ+m887Z5z7PzGuzX/lnfW+aUhjkbo41ltDBlAPC4PPuSwHfRF+j0AQwLlpNFoAf0Jhxt5u3/T7hxALU9gh5GnxZijudW50acUY3xxm00n6zmUzS5RMOqE2tPGZvJmMZGISPaJzOjnLYA5HaSqwHSsHD5qGSLXLMC1VED1Sy9lypXRrugR1QcHFXzBTOIUw1qHSqBnCv76Bri+Y4CdF4601dBMF6cFLWWW/s2N/e62aye1YQhnw3o/ljsb1UBF2fYAXAf HsL/plQs 0zij2wnBBcYtrem+EQVBtrFtj4AYDjh2pFqcE28MAhWSo+XIC2tNBe7hjyffb/C3idcwlTyW7dcVV9FmOV4ItgPj1R8E7zvtTwa5xFgVKtuxVW1zd0+HgCVVgMuFavkke0qZ9h2j4IXJkNu0Cui/Q0jgxoBbSeQ3Qn5FgYlACjek40tqqrprCFbNQDKH7lAY8l+0K18s7/2yqnrps1EPBuijaoLSxAJAWtsBf3wT0xTdjFjmjkcFkqYmTchTFUrHArqYHdlp7UhYg2My5Azk2QjhhmKQzDrKnsWf97nq3IlGhbBOXPhc61QwITuw9Ki033UYAoRnO9TXD6i/C2aNc0JNvpUCQA+nuXzVbjkupue6u2mo= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/11/26 08:57, Wandun Chen wrote: > From: Chen Wandun > > madvise_collapse() computes the THP-aligned window: > > hstart = ALIGN(start, HPAGE_PMD_SIZE); /* round up */ > hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE); /* round down */ > > The following case will cause hstart > hend, and result in underflow > in the return statement, avoid it by returning -EINVAL early when > hstart > hend. > > madvise(PMD-aligned + PAGE_SIZE, PAGE_SIZE, MADV_COLLAPSE); Ok, so providing a PMD-aligned address as start will result in 0 and a non-aligned address will result in -EINVAL. Didn't Lorenzo agree that just returning 0 in both cases would be clearer? But I might have misunderstood it. That would also make the code easier ... > > In addition, kmalloc_obj(), mmgrab() and lru_add_drain_all() are > unnecessary when hstart == hend, so skip these operations by > returning early too. > > Signed-off-by: Chen Wandun > > --- > v1 --> v2: > - Rebase and resolve code conflict. > - Return -EINVAL when hstart > hend, suggested by Lorenzo. > - Drop Fixes tag, suggested by David and Lorenzo. > - Updated commit message to be more explicit, suggested by Lorenzo. > --- > mm/khugepaged.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 28a843f30b32..36baab17f098 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2837,6 +2837,15 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, > if (!thp_vma_allowable_order(vma, vma->vm_flags, TVA_FORCED_COLLAPSE, PMD_ORDER)) > return -EINVAL; > > + hstart = ALIGN(start, HPAGE_PMD_SIZE); > + hend = ALIGN_DOWN(end, HPAGE_PMD_SIZE); > + > + if (hstart > hend) > + return -EINVAL; > + > + if (hstart == hend) > + return 0; > + ... as it would simply be: /* Nothing to collapse. */ if (hstart >= hend) return 0; @Lorenzo, or do we want to keep the (questionable : ) ) existing behavior? -- Cheers, David