linux-kernel - Re: [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race

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Date:	Tue, 28 Feb 2012 20:17:28 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	mgorman@...e.de, kamezawa.hiroyu@...fujitsu.com,
	viro@...iv.linux.org.uk, hughd@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race

On Tue, Feb 28, 2012 at 6:15 PM, Aneesh Kumar K.V
<aneesh.kumar@...ux.vnet.ibm.com> wrote:
>
> Will update the patch with these details
>

A scratch is cooked, based on the -next tree, for accelerating your redelivery,
if you like it, in which i_mutex is eliminated directly and page lock is used.

-hd


--- a/fs/hugetlbfs/inode.c	Tue Feb 28 19:43:32 2012
+++ b/fs/hugetlbfs/inode.c	Tue Feb 28 19:56:50 2012
@@ -245,17 +245,10 @@ static ssize_t hugetlbfs_read(struct fil
 	loff_t isize;
 	ssize_t retval = 0;

-	mutex_lock(&inode->i_mutex);
-
 	/* validate length */
 	if (len == 0)
 		goto out;

-	isize = i_size_read(inode);
-	if (!isize)
-		goto out;
-
-	end_index = (isize - 1) >> huge_page_shift(h);
 	for (;;) {
 		struct page *page;
 		unsigned long nr, ret;
@@ -263,6 +256,8 @@ static ssize_t hugetlbfs_read(struct fil

 		/* nr is the maximum number of bytes to copy from this page */
 		nr = huge_page_size(h);
+		isize = i_size_read(inode);
+		end_index = isize >> huge_page_shift(h);
 		if (index >= end_index) {
 			if (index > end_index)
 				goto out;
@@ -274,7 +269,7 @@ static ssize_t hugetlbfs_read(struct fil
 		nr = nr - offset;

 		/* Find the page */
-		page = find_get_page(mapping, index);
+		page = find_lock_page(mapping, index);
 		if (unlikely(page == NULL)) {
 			/*
 			 * We have a HOLE, zero out the user-buffer for the
@@ -286,17 +281,30 @@ static ssize_t hugetlbfs_read(struct fil
 			else
 				ra = 0;
 		} else {
+			unlock_page(page);
+
+			/* Without i_mutex held, check isize again */
+			nr = huge_page_size(h);
+			isize = i_size_read(inode);
+			end_index = isize >> huge_page_shift(h);
+			if (index == end_index) {
+				nr = isize & ~huge_page_mask(h);
+				if (nr <= offset) {
+					page_cache_release(page);
+					goto out;
+				}
+			}
+			nr -= offset;
 			/*
 			 * We have the page, copy it to user space buffer.
 			 */
 			ra = hugetlbfs_read_actor(page, offset, buf, len, nr);
 			ret = ra;
+			page_cache_release(page);
 		}
 		if (ra < 0) {
 			if (retval == 0)
 				retval = ra;
-			if (page)
-				page_cache_release(page);
 			goto out;
 		}

@@ -306,16 +314,12 @@ static ssize_t hugetlbfs_read(struct fil
 		index += offset >> huge_page_shift(h);
 		offset &= ~huge_page_mask(h);

-		if (page)
-			page_cache_release(page);
-
 		/* short read or no more work */
 		if ((ret != nr) || (len == 0))
 			break;
 	}
 out:
 	*ppos = ((loff_t)index << huge_page_shift(h)) + offset;
-	mutex_unlock(&inode->i_mutex);
 	return retval;
 }

--
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