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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=RJhg8HWY73+ZEkyEoyDbcVYjsCE09nGUo617LE3EVXg=; b=Ek8CY5B/oagxK7EGDezEcocJUfbX/6z/YaTRmNol8p+btOf8KDTNbB515+f6yOoha9 rInoSFwZz1ktozH42JE08/SNGU7WiaUBVjX0uk/fm1R3hwXa1HQfNgOlrJvtzV3HBOtP PiLBvqPzax2f+Wqb2o4pNdjhG9gMcvrTTN46UCUwlMmAw/1AvEpV+9TvAWqZDGzXMMG4 Ttn72fTklm3o2b4s/0p96XqY83CLpRNiX+PqY48UuRPMe+h1evI5BwF7o/cSZVFfLrfo +jWhuz4JudW4FbAbAowmjZM3vl/L5hmLEpd2NgxRtSnCOnuO9GeH6M5HS/CpET12aip9 74dA== X-Gm-Message-State: AO0yUKWNZgo/katelc27BkPZp7CwyAGHWTwqMFpw07DYP+oZnHkkHJHO 2+6B3PGfN9BgvtkTC3R+j7/x/D0vw2Lkym/omIc= X-Google-Smtp-Source: AK7set8eyCwuwnpxHr9Cb5IrHAhzqgGXMkGPE2szqGUbIaxgkfmlbNTfJaxIJMYCjD35g51ztYgjh9Ar2jl4XnUdzOE= X-Received: by 2002:aa7:9987:0:b0:590:7611:1c4a with SMTP id k7-20020aa79987000000b0059076111c4amr166067pfh.63.1674670004787; Wed, 25 Jan 2023 10:06:44 -0800 (PST) MIME-Version: 1.0 References: <20230125015738.912924-1-zokeefe@google.com> In-Reply-To: <20230125015738.912924-1-zokeefe@google.com> From: Yang Shi Date: Wed, 25 Jan 2023 10:06:33 -0800 Message-ID: Subject: Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount To: "Zach O'Keefe" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Hugh Dickins Content-Type: text/plain; charset="UTF-8" X-Rspamd-Queue-Id: 2151A4000C X-Rspamd-Server: rspam09 X-Rspam-User: X-Stat-Signature: bg98u95wz6gds73t77orjact73ufn89i X-HE-Tag: 1674670005-36277 X-HE-Meta: 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 UYbelPsu 47m0lWX1hq8kdChWCPCK2nslBhK5V/u/AH46C6rfxfH3JHpw+FkD7+wuxu83/AbmRSmunMmB93FuXQa50Kq3EPBzrHX3zef4J6+1YSlVrRANBDUtIwxG+0BWPzuEtTffzGZWUBN/9nOkqeDpInZndiFnpqOfEiYKf/yAZ+tdtV/e/5C4rNS4zGo8X6alP7r/Ev9Gtcvd3m4y1uq78LXKtxBEwhiZS9hgXVINzDAlEySb/fN1kzkgYY8b5QNn+sPnQ5ZEOj0fOum2GBJdSO/AlYLBVZXHntNWyAJbAf3Isz8knXqWw+5q3eHMntfdXMFLbNFvrhefW27t8E/KvSgIk3eeH39SQf968auror3Bw5r2S42fx8aFZxfJSs51Hlxy81m2v3H1RLmDUQOUpdseF2Hqo96AFWs2wW2U2ubz5C6Dw4IWSE/dhbc+yZN9DbdCvu8b7T1/60q0t8Bs= 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: On Tue, Jan 24, 2023 at 5:58 PM Zach O'Keefe wrote: > > During collapse, in a few places we check to see if a given small page > has any unaccounted references. If the refcount on the page doesn't > match our expectations, it must be there is an unknown user concurrently > interested in the page, and so it's not safe to move the contents > elsewhere. However, the unaccounted pins are likely an ephemeral state. > > In such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that > collapse may succeed on retry. The page may be DMA pinned (for example, pin_user_pages()), it is not worth retrying for such pages. But it may also not be worth optimizing for this case at this point. So the patch looks good to me. Reviewed-by: Yang Shi > > Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse") > Reported-by: Hugh Dickins > Signed-off-by: Zach O'Keefe > > --- > mm/khugepaged.c | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index e23619bfecc4..fa38cae240b9 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -2712,6 +2712,7 @@ static int madvise_collapse_errno(enum scan_result r) > case SCAN_CGROUP_CHARGE_FAIL: > return -EBUSY; > /* Resource temporary unavailable - trying again might succeed */ > + case SCAN_PAGE_COUNT: > case SCAN_PAGE_LOCK: > case SCAN_PAGE_LRU: > case SCAN_DEL_PAGE_LRU: > -- > 2.39.1.405.gd4c25cc71f-goog >