Messages in this thread | | | Date | Mon, 11 Feb 2008 13:15:30 +0300 | From | Oleg Nesterov <> | Subject | Re: [PATCH] sys_remap_file_pages: fix ->vm_file accounting |
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Sorry for delay,
On 02/06, Hugh Dickins wrote: > > On Sun, 3 Feb 2008, Oleg Nesterov wrote: > > Off-topic question to all. sys_remap_file_pages() doesn't work with > > shared readonly mappings, why? > > Slight correction: it works with shared readonly mappings, doesn't > it, so long as the mmap'ed file was opened for reading and writing?
Yes sure. I meant, if the file was opened without FMODE_WRITE, then mmap(PROT_READ, MAP_SHARED) doesn't actually set VM_SHARED, it only sets VM_MAYSHARE.
(this looks understandable, but means that !VM_SHARED doesn't necessary imply the possible cow).
> > IOW, why it checks VM_SHARED but not VM_MAYSHARE? > > My guess has always been that it was just a misunderstanding of how > those VM_ flags end up working: assume so unless Ingo corrects me. > > By the time I realized that oddity, we'd been driven into several > tiresome corners by the very existence of sys_remap_file_pages. > So whereas my usual instinct would have been to relax the restriction > and generalize, in its case I wanted to hold on to every restriction > we had.
OK, thanks a lot. I was afraid I missed some "obvious" reason why we can't do this.
Oleg.
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