iov_iter: saner helper for page array allocation

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

Returns number of slots available in resulting array or 0 on OOM;
it's up to the caller to make sure it doesn't ask to zero-entry
array (i.e. neither maxpages nor size are allowed to be zero).

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index a9446ef..f003a20 100644 (file)
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1284,9 +1284,20 @@ 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 int count = DIV_ROUND_UP(size + start, PAGE_SIZE);
+
+       if (count > maxpages)
+               count = maxpages;
+       WARN_ON(!count);        // caller should've prevented that
+       if (!*res) {
+               *res = kvmalloc_array(count, sizeof(struct page *), GFP_KERNEL);
+               if (!*res)
+                       return 0;
+       }
+       return count;
 }
 
 static ssize_t pipe_get_pages(struct iov_iter *i,
@@ -1294,27 +1305,20 @@ static ssize_t pipe_get_pages(struct iov_iter *i,
                   size_t *start)
 {
        struct pipe_inode_info *pipe = i->pipe;
-       unsigned int npages, off;
+       unsigned int npages, off, count;
        struct page **p;
        ssize_t left;
-       int count;
 
        if (!sanity(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;
+       if (!npages)
+               return -EFAULT;
+       count = want_pages_array(pages, maxsize, off, min(npages, maxpages));
+       if (!count)
+               return -ENOMEM;
        p = *pages;
-       if (!p) {
-               *pages = p = get_pages_array(count);
-               if (!p)
-                       return -ENOMEM;
-       }
-
        left = maxsize;
        npages = 0;
        if (off) {
@@ -1377,9 +1381,8 @@ static ssize_t iter_xarray_get_pages(struct iov_iter *i,
                                     struct page ***pages, size_t maxsize,
                                     unsigned maxpages, size_t *_start_offset)
 {
-       unsigned nr, offset;
-       pgoff_t index, count;
-       size_t size = maxsize;
+       unsigned nr, offset, count;
+       pgoff_t index;
        loff_t pos;
 
        pos = i->xarray_start + i->iov_offset;
@@ -1387,16 +1390,9 @@ 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)
+               return -ENOMEM;
        nr = iter_xarray_populate_pages(*pages, i->xarray, index, count);
        if (nr == 0)
                return 0;
@@ -1445,7 +1441,7 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
                   struct page ***pages, size_t maxsize,
                   unsigned int maxpages, size_t *start)
 {
-       int n, res;
+       unsigned int n;
 
        if (maxsize > i->count)
                maxsize = i->count;
@@ -1457,6 +1453,7 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
        if (likely(user_backed_iter(i))) {
                unsigned int gup_flags = 0;
                unsigned long addr;
+               int res;
 
                if (iov_iter_rw(i) != WRITE)
                        gup_flags |= FOLL_WRITE;
@@ -1466,14 +1463,9 @@ 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)
+                       return -ENOMEM;
                res = get_user_pages_fast(addr, n, gup_flags, *pages);
                if (unlikely(res <= 0))
                        return res;
@@ -1484,15 +1476,10 @@ 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)
+                       return -ENOMEM;
                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);