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