Messages in this thread | | | Date | Sat, 30 Nov 2013 15:52:45 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] Add a text_poke syscall v2 |
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On 11/29, H. Peter Anvin wrote: > > On 11/29/2013 12:05 PM, Oleg Nesterov wrote: > > > > Can't we invalidate pte (so that any user will stuck in page fault), > > update the page(s), restore the pte and drop the locks? > > > > This way sys_text_poke() won't be x86-specific, and it will be per-mm. > > > > Hmmm... if we hold mmap_sem() this pretty much will be the net result, > no?
Yes, down_write(mmap_sem) is enough to block the page faults.
But we need pte-lock anyway, to avoid the races with, say, try_to_unmap(). (and of course, we need to retry if page_check_address() fails).
This actually means that if we want to update a single page we could use down_read(). But in general we need to update 2 pages. Or even more if we generalize sys_text_poke(), perhaps it should be renamed in this case, but this is off-topic.
> That would mean no additional tests needed on the page fault path.
Not sure I understand... but of course we should not change the fault paths in any case.
> What I'm not sure of is whether or not it is actually safe to hold > mmap_sem across all the code we need
Let me repeat, I do not understand vm enough to answer authoritatively.
But I think this should be safe. I do not see why it should not, but see above.
Oleg.
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