Messages in this thread | | | Date | Wed, 2 Mar 2022 16:59:34 -0400 | From | Jason Gunthorpe <> | Subject | Re: [PATCH RFC 12/13] mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page |
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On Wed, Mar 02, 2022 at 09:38:09PM +0100, David Hildenbrand wrote:
> (a) I want a R/O pin to observe file modifications. > (b) I want the R/O pin to *not* observe file modifications but observe > my (eventual? if any) private modifications,
A scenario I know that motivated this is fairly straightfoward:
static char data[] = {};
ibv_reg_mr(data, READ_ONLY) data[0] = 1 .. go touch data via DMA ..
We want to reliably observe the '1'
What is happening under the covers is that 'data' is placed in the .data segment and becomes a file backed MAP_PRIVATE page. The write COWs that page
It would work OK if it was in .bss instead
I think the FOLL_FORCE is there because otherwise the trick doesn't work on true RO pages and the API becomes broken.
Jason
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