Re: [PATCH] sys_remap_file_pages: fix ->vm_file accounting

[Hugh Dickins]

[ removed stable ]

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?

> 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.

sys_remap_file_pages does serve a useful purpose; but it subverts
vma principles, and has therefore caused us grief repeatedly.

Hugh
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