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SubjectRe: [PATCH] Alternate futex non-page-pinning and COW fix
Linus Torvalds wrote:
> The only person who should ever test VM_MAYSHARE is somebody who does
> reporting back to user space: VM_MAYSHARE basically ends up meaning "the
> user _asked_ for a shared mapping". While "VM_SHARED" means "this mapping
> can actually contain a shared dirty page".

Precisely. And for futex, the correct meaning is "the user asked for
a shared mapping": a FUTEX_WAIT on a shared mapping of a file opened
read-only _should_ be woken by a FUTEX_WAKE on the same file by
another process.

> The VM itself should only ever care about VM_SHARED. Because that's the
> only bit that has real semantic meaning.

remap_file_pages() returns -EINVAL on mappings of
read-only file handles because it tests VM_SHARED. There is no good
reason not to remap a read-only file - it doesn't complain about
remapping a PROT_READ mapping of a writable file!

Patch tested and attached.

Enjoy,
-- Jamie

Subject: [PATCH] Allow remap_file_pages() on read-only files
Patch: nonlinear-mayshare-2.6.0-test4-02.2jl

This changes remap_file_pages() to work on mappings of read-only file
handles. Without this it returns -EINVAL.

diff -urN --exclude-from=dontdiff orig-2.6.0-test4/mm/fremap.c nonlinear-2.6.0-test4/mm/fremap.c
--- orig-2.6.0-test4/mm/fremap.c 2003-07-08 21:44:29.000000000 +0100
+++ nonlinear-2.6.0-test4/mm/fremap.c 2003-09-04 18:19:45.000000000 +0100
@@ -145,10 +145,11 @@
vma = find_vma(mm, start);
/*
* Make sure the vma is shared, that it supports prefaulting,
- * and that the remapped range is valid and fully within
- * the single existing vma:
+ * and that the remapped range is valid and fully within the
+ * single existing vma. VM_MAYSHARE is checked (not VM_SHARED)
+ * so that read-only files can be remapped too:
*/
- if (vma && (vma->vm_flags & VM_SHARED) &&
+ if (vma && (vma->vm_flags & VM_MAYSHARE) &&
vma->vm_ops && vma->vm_ops->populate &&
end > start && start >= vma->vm_start &&
end <= vma->vm_end)
-
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