* RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage.
@ 2002-11-08 22:49 David Woodhouse
2002-11-08 23:31 ` Linus Torvalds
2002-11-08 23:50 ` David Woodhouse
0 siblings, 2 replies; 5+ messages in thread
From: David Woodhouse @ 2002-11-08 22:49 UTC (permalink / raw)
To: linux-kernel
Why does a _readonly_ mapping fail if the file system has no writepage
method?
do_mmap_pgoff() sets VM_MAYWRITE on the vma and then generic_file_mmap()
refuses to allow it.
Suggested patch below.... or should I just hack fsx-linux to use
MAP_PRIVATE for its readonly mappings and ignore it?
--- 1.157/mm/filemap.c Sun Nov 3 02:55:27 2002
+++ edited/mm/filemap.c Fri Nov 8 22:08:22 2002
@@ -1311,9 +1311,12 @@
struct address_space *mapping = file->f_dentry->d_inode->i_mapping;
struct inode *inode = mapping->host;
- if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE)) {
- if (!mapping->a_ops->writepage)
+ if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE) &&
+ !mapping->a_ops->writepage) {
+ if (vma->vm_flags & VM_WRITE)
return -EINVAL;
+ else
+ vma->vm_flags &= ~VM_MAYWRITE;
}
if (!mapping->a_ops->readpage)
return -ENOEXEC;
--
dwmw2
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage.
2002-11-08 22:49 RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage David Woodhouse
@ 2002-11-08 23:31 ` Linus Torvalds
2002-11-08 23:50 ` David Woodhouse
1 sibling, 0 replies; 5+ messages in thread
From: Linus Torvalds @ 2002-11-08 23:31 UTC (permalink / raw)
To: linux-kernel
In article <24305.1036795742@passion.cambridge.redhat.com>,
David Woodhouse <dwmw2@infradead.org> wrote:
>Why does a _readonly_ mapping fail if the file system has no writepage
>method?
>
>do_mmap_pgoff() sets VM_MAYWRITE on the vma and then generic_file_mmap()
>refuses to allow it.
>
>Suggested patch below.... or should I just hack fsx-linux to use
>MAP_PRIVATE for its readonly mappings and ignore it?
This is broken. Since it has VM_MAYWRITE, a subsequent mprotect() may
mark it writable, and you you went boom.
If you really want a shared mapping, you'd better open with O_RDONLY, at
which point the existing code should be perfectly happy and does the
right thing.
In other words: the code is correct as-is.
Linus
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage.
2002-11-08 22:49 RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage David Woodhouse
2002-11-08 23:31 ` Linus Torvalds
@ 2002-11-08 23:50 ` David Woodhouse
2002-11-09 2:46 ` Linus Torvalds
1 sibling, 1 reply; 5+ messages in thread
From: David Woodhouse @ 2002-11-08 23:50 UTC (permalink / raw)
To: Linus Torvalds; +Cc: linux-kernel
torvalds@transmeta.com said:
> This is broken. Since it has VM_MAYWRITE, a subsequent mprotect() may
> mark it writable, and you you went boom.
Er, we clear VM_MAYWRITE...
+ if (vma->vm_flags & VM_WRITE)
return -EINVAL;
+ else
+ vma->vm_flags &= ~VM_MAYWRITE;
> If you really want a shared mapping, you'd better open with O_RDONLY,
> at which point the existing code should be perfectly happy and does
> the right thing.
It's a read-only mapping. Whether it's shared or private is not relevant,
surely, since those affect only the behaviour if we write to it -- which we
can't.
I don't _really_ want a shared mapping; all I want is for the fsx-linux
stress test to run, and find interesting breakage on my file system to keep
me from getting bored (what are Friday nights for, after all?).
As shipped, fsx-linux uses PROT_READ, MAP_SHARED on its test file, which
definitely needs to be opened for write. For now, I've just changed it to
use MAP_PRIVATE. I'm just a bit concerned about having to change the test to
get it to work though, and don't see why a _readonly_ mmap should fail due
to lack of writepage.
--
dwmw2
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage.
2002-11-08 23:50 ` David Woodhouse
@ 2002-11-09 2:46 ` Linus Torvalds
2002-11-09 16:36 ` David Woodhouse
0 siblings, 1 reply; 5+ messages in thread
From: Linus Torvalds @ 2002-11-09 2:46 UTC (permalink / raw)
To: David Woodhouse; +Cc: linux-kernel
On Fri, 8 Nov 2002, David Woodhouse wrote:
>
> It's a read-only mapping. Whether it's shared or private is not relevant,
> surely, since those affect only the behaviour if we write to it -- which we
> can't.
But why do you expect to be able to mmap shared something you opened RW in
the first place, if the filesystem cannot handle writable m appings?
> I don't _really_ want a shared mapping; all I want is for the fsx-linux
> stress test to run, and find interesting breakage on my file system to keep
> me from getting bored (what are Friday nights for, after all?).
Ok, it's good to have tests, but it sounds like the test is broken.
> As shipped, fsx-linux uses PROT_READ, MAP_SHARED on its test file
Ad shipped, the kernel doesn't allow that.
Notice the pattern here? "As shipped". You have two choices: make
gratuitous changes to the kernel to make some random test happy, or fix
the test.
I think you should fix the test. The kernel change buys you _zero_ new
features.
Linus
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage.
2002-11-09 2:46 ` Linus Torvalds
@ 2002-11-09 16:36 ` David Woodhouse
0 siblings, 0 replies; 5+ messages in thread
From: David Woodhouse @ 2002-11-09 16:36 UTC (permalink / raw)
To: Linus Torvalds; +Cc: linux-kernel
dwmw2@infradead.org said:
> Suggested patch below.... or should I just hack fsx-linux to use
> MAP_PRIVATE for its readonly mappings and ignore it?
torvalds@transmeta.com said:
> You have two choices: make gratuitous changes to the kernel to make
> some random test happy, or fix the test.
> I think you should fix the test. The kernel change buys you _zero_ new
> features.
Works for me. Thanks for the answer.
--
dwmw2
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2002-11-09 16:30 UTC | newest]
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2002-11-08 22:49 RFC: mmap(PROT_READ, MAP_SHARED) fails if !writepage David Woodhouse
2002-11-08 23:31 ` Linus Torvalds
2002-11-08 23:50 ` David Woodhouse
2002-11-09 2:46 ` Linus Torvalds
2002-11-09 16:36 ` David Woodhouse
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