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From:   Charan Teja Kalla <quic_charante@quicinc.com>
To:     <akpm@linux-foundation.org>, <surenb@google.com>, <vbabka@suse.cz>,
        <rientjes@google.com>, <sfr@canb.auug.org.au>,
        <edgararriaga@google.com>, <minchan@kernel.org>,
        <nadav.amit@gmail.com>, <mhocko@suse.com>
CC:     <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>,
        Charan Teja Kalla <quic_charante@quicinc.com>,
        "# 5 . 10+" <stable@vger.kernel.org>
Subject: [PATCH V2,2/2] mm: madvise: skip unmapped vma holes passed to process_madvise
Date:   Fri, 11 Mar 2022 20:59:06 +0530
Message-ID: <4f091776142f2ebf7b94018146de72318474e686.1647008754.git.quic_charante@quicinc.com>
In-Reply-To: <cover.1647008754.git.quic_charante@quicinc.com>
References: <cover.1647008754.git.quic_charante@quicinc.com>
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The process_madvise() system call is expected to skip holes in vma
passed through 'struct iovec' vector list. But do_madvise, which
process_madvise() calls for each vma, returns ENOMEM in case of unmapped
holes, despite the VMA is processed.
Thus process_madvise() should treat ENOMEM as expected and consider the
VMA passed to as processed and continue processing other vma's in the
vector list. Returning -ENOMEM to user, despite the VMA is processed,
will be unable to figure out where to start the next madvise.

Fixes: ecb8ac8b1f14("mm/madvise: introduce process_madvise() syscall: an external memory hinting API")
Cc: <stable@vger.kernel.org> # 5.10+
Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
---
Changes in V2:
  -- Fixed handling of ENOMEM by process_madvise().
  -- Patch doesn't exist in V1.

 mm/madvise.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/madvise.c b/mm/madvise.c
index e97e6a9..14fb76d 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -1426,9 +1426,16 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
 
 	while (iov_iter_count(&iter)) {
 		iovec = iov_iter_iovec(&iter);
+		/*
+		 * do_madvise returns ENOMEM if unmapped holes are present
+		 * in the passed VMA. process_madvise() is expected to skip
+		 * unmapped holes passed to it in the 'struct iovec' list
+		 * and not fail because of them. Thus treat -ENOMEM return
+		 * from do_madvise as valid and continue processing.
+		 */
 		ret = do_madvise(mm, (unsigned long)iovec.iov_base,
 					iovec.iov_len, behavior);
-		if (ret < 0)
+		if (ret < 0 && ret != -ENOMEM)
 			break;
 		iov_iter_advance(&iter, iovec.iov_len);
 	}
-- 
2.7.4