Re: [PATCH 22/31] iov_iter: saner helper for page array allocation

[Al Viro <viro@zeniv.linux.org.uk>]

[braino fix]
From e64d637d648390e4ac0643747ae174c3be15f243 Mon Sep 17 00:00:00 2001
From: Al Viro <viro@zeniv.linux.org.uk>
Date: Fri, 17 Jun 2022 14:45:41 -0400
Subject: [PATCH 22/31] iov_iter: saner helper for page array allocation

All call sites of get_pages_array() are essenitally identical now.
Replace with common helper...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 lib/iov_iter.c | 64 +++++++++++++++++++-------------------------------
 1 file changed, 24 insertions(+), 40 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index a65c766936cc..2ce062f1817d 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1187,9 +1187,19 @@ unsigned long iov_iter_gap_alignment(const struct iov_iter *i)
 }
 EXPORT_SYMBOL(iov_iter_gap_alignment);
 
-static struct page **get_pages_array(size_t n)
+static int want_pages_array(struct page ***res, size_t size,
+			    size_t start, unsigned int maxpages)
 {
-	return kvmalloc_array(n, sizeof(struct page *), GFP_KERNEL);
+	unsigned count = DIV_ROUND_UP(size + start, PAGE_SIZE);
+
+	if (count > maxpages)
+		count = maxpages;
+	if (!*res) {
+		*res = kvmalloc_array(count, sizeof(struct page *), GFP_KERNEL);
+		if (!*res)
+			return -ENOMEM;
+	}
+	return count;
 }
 
 static ssize_t pipe_get_pages(struct iov_iter *i,
@@ -1206,18 +1216,10 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
 		return -EFAULT;
 
 	*start = off = pipe_npages(i, &npages);
-	count = DIV_ROUND_UP(maxsize + off, PAGE_SIZE);
-	if (count > npages)
-		count = npages;
-	if (count > maxpages)
-		count = maxpages;
+	count = want_pages_array(pages, maxsize, off, min(npages, maxpages));
+	if (count < 0)
+		return count;
 	p = *pages;
-	if (!p) {
-		*pages = p = get_pages_array(count);
-		if (!p)
-			return -ENOMEM;
-	}
-
 	left = maxsize;
 	npages = 0;
 	if (off) {
@@ -1282,7 +1284,6 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
 {
 	unsigned nr, offset;
 	pgoff_t index, count;
-	size_t size = maxsize;
 	loff_t pos;
 
 	pos = i->xarray_start + i->iov_offset;
@@ -1290,16 +1291,9 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
 	offset = pos & ~PAGE_MASK;
 	*_start_offset = offset;
 
-	count = DIV_ROUND_UP(size + offset, PAGE_SIZE);
-	if (count > maxpages)
-		count = maxpages;
-
-	if (!*pages) {
-		*pages = get_pages_array(count);
-		if (!*pages)
-			return -ENOMEM;
-	}
-
+	count = want_pages_array(pages, maxsize, offset, maxpages);
+	if (count < 0)
+		return count;
 	nr = iter_xarray_populate_pages(*pages, i->xarray, index, count);
 	if (nr == 0)
 		return 0;
@@ -1369,14 +1363,9 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
 		addr = first_iovec_segment(i, &maxsize);
 		*start = addr % PAGE_SIZE;
 		addr &= PAGE_MASK;
-		n = DIV_ROUND_UP(maxsize + *start, PAGE_SIZE);
-		if (n > maxpages)
-			n = maxpages;
-		if (*pages) {
-			*pages = get_pages_array(n);
-			if (!*pages)
-				return -ENOMEM;
-		}
+		n = want_pages_array(pages, maxsize, *start, maxpages);
+		if (n < 0)
+			return n;
 		res = get_user_pages_fast(addr, n, gup_flags, *pages);
 		if (unlikely(res <= 0))
 			return res;
@@ -1387,15 +1376,10 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
 		struct page *page;
 
 		page = first_bvec_segment(i, &maxsize, start);
-		n = DIV_ROUND_UP(maxsize + *start, PAGE_SIZE);
-		if (n > maxpages)
-			n = maxpages;
+		n = want_pages_array(pages, maxsize, *start, maxpages);
+		if (n < 0)
+			return n;
 		p = *pages;
-		if (!p) {
-			*pages = p = get_pages_array(n);
-			if (!p)
-				return -ENOMEM;
-		}
 		for (int k = 0; k < n; k++)
 			get_page(*p++ = page++);
 		return min_t(size_t, maxsize, n * PAGE_SIZE - *start);
-- 
2.30.2