Re: [PATCH RFC] mm: account VMA before forced-COW via /proc/pid/mem

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

On 04/09, Hugh Dickins wrote:
> Let me reiterate here that I was off at a tangent in bringing this up,
> so sorry for any confusion I spread.

I guess it was me who added the confusion ;)

> > OTOH, if the file was opened without FMODE_WRITE, then I do not
> > really understand how (PROT_READ, MAP_SHARED) differs from

I meant, from gup(FOLL_FORCE|FOLL_WRITE) pov. I didn't mean mprotect/etc.

> The strange weird confusing part is that having checked that you have
> permission to write to the file, it then avoids doing so (unless the
> area currently has PROT_WRITE): it COWs pages for you instead,
> leaving unexpected anon pages in the shared area.

Yes, and we could do the same in (PROT_READ, MAP_SHARED) case.

This is what looks strange to me. We require PROT_WRITE to force-
cow, although we are not going (and shouldn't) write to the file.

But, to avoid even more confusion, I am not arguing with your
"limit the damage by making GUP write,force fail in that case"
suggestion. At least I do not think ptrace/gdb can suffer.

> > Speaking of the difference above, I'd wish I could understand
> > what VM_MAYSHARE actually means except "MAP_SHARED was used".
> That's precisely it: so it's very useful in /proc/pid/maps, for
> deciding whether to show an 's' or a 'p', but not so often when
> real decisions are made (where, as you've observed, private readonly
> and shared readonly are treated very similarly, without VM_SHARED).

Aha, thanks a lot.


To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx.  For more info on Linux MM,
see: .
Fight unfair telecom internet charges in Canada: sign
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>

[Site Home]     [Linux ARM Kernel]     [Linux ARM]     [Linux Omap]     [Fedora ARM]     [IETF Annouce]     [Security]     [Bugtraq]     [Linux]     [Linux OMAP]     [Linux MIPS]     [ECOS]     [Tools]     [DDR & Rambus]     [Asterisk Internet PBX]     [Linux API]     [Monitors]

Add to Google Google PageRank Checking tool