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From: Souptick Joarder <>
	Souptick Joarder <>
Subject: [PATCH] xen/gntdev.c: Replace vm_map_pages() with vm_map_pages_zero()
Date: Wed, 31 Jul 2019 00:04:56 +0530	[thread overview]
Message-ID: <> (raw)

'commit df9bde015a72 ("xen/gntdev.c: convert to use vm_map_pages()")'
breaks gntdev driver. If vma->vm_pgoff > 0, vm_map_pages()
 - use map->pages starting at vma->vm_pgoff instead of 0
 - verify map->count against vma_pages()+vma->vm_pgoff instead of just

In practice, this breaks using a single gntdev FD for mapping multiple

relevant strace output:
[pid   857] ioctl(7, IOCTL_GNTDEV_MAP_GRANT_REF, 0x7ffd3407b6d0) = 0
[pid   857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7, 0) =
[pid   857] ioctl(7, IOCTL_GNTDEV_SET_UNMAP_NOTIFY, 0x7ffd3407b710) = 0
[pid   857] ioctl(7, IOCTL_GNTDEV_MAP_GRANT_REF, 0x7ffd3407b6d0) = 0
[pid   857] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_SHARED, 7,
0x1000) = -1 ENXIO (No such device or address)

details here:

The reason is -> ( copying Marek's word from discussion)

vma->vm_pgoff is used as index passed to gntdev_find_map_index. It's
basically using this parameter for "which grant reference to map".
map struct returned by gntdev_find_map_index() describes just the pages
to be mapped. Specifically map->pages[0] should be mapped at
vma->vm_start, not vma->vm_start+vma->vm_pgoff*PAGE_SIZE.

When trying to map grant with index (aka vma->vm_pgoff) > 1,
__vm_map_pages() will refuse to map it because it will expect map->count
to be at least vma_pages(vma)+vma->vm_pgoff, while it is exactly

Converting vm_map_pages() to use vm_map_pages_zero() will fix the

Marek has tested and confirmed the same.

Reported-by: Marek Marczykowski-Górecki <>
Signed-off-by: Souptick Joarder <>
Tested-by: Marek Marczykowski-Górecki <>
 drivers/xen/gntdev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 4c339c7..a446a72 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -1143,7 +1143,7 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
 		goto out_put_map;
 	if (!use_ptemod) {
-		err = vm_map_pages(vma, map->pages, map->count);
+		err = vm_map_pages_zero(vma, map->pages, map->count);
 		if (err)
 			goto out_put_map;
 	} else {

             reply	other threads:[~2019-07-30 18:32 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-30 18:34 Souptick Joarder [this message]
2019-07-30 19:56 ` [PATCH] xen/gntdev.c: Replace vm_map_pages() with vm_map_pages_zero() Boris Ostrovsky
2019-07-31  6:15 ` Juergen Gross

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