Messages in this thread | | | Date | Thu, 20 Dec 2007 15:26:00 +0000 (GMT) | From | Hugh Dickins <> | Subject | Re: [rfc][patch] mm: madvise(WILLNEED) for anonymous memory |
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On Thu, 20 Dec 2007, Peter Zijlstra wrote: > On Thu, 2007-12-20 at 14:09 +0000, Hugh Dickins wrote: > > On Thu, 20 Dec 2007, Peter Zijlstra wrote: > > > > I certainly agree with this in principle: it just seems an unnecessary > > and surprising restriction to refuse on anonymous vmas; I guess the only > > reason for not adding this was not having anyone asking for it until now. > > Though, does Lennart realize he could use MAP_POPULATE in the mmap? > > I think he's trying to get his data swapped-in.
That's perfectly reasonable, fair enough.
> > > +{ > > > + int ret, len; > > > + > > > + *prev = vma; > > > + if (end > vma->vm_end) > > > + end = vma->vm_end; > > > > Please check, but I think the upper level ensures end is within range. > > It certainly looks like it, but I since the file case did this check I > thought it prudent to also do it. I guess I might as well remove both.
Ah, so it does. Yes, please do remove both.
> > Hmm, might it be better to use make_pages_present itself, > > fixing its retval, rather than using get_user_pages directly? > > (I'd hope the caching makes its repeat of find_vma not an overhead.) > > > > Interesting divergence: make_pages_present faults in writable pages > > in a writable vma, whereas the file case's force_page_cache_readahead > > doesn't even insert the pages into the mm. > > Yeah, the find_vma and write fault thing are the reason I didn't use > make_pages_present.
The write fault thing is irrelevant now, actually: now do_anonymous_page doesn't use ZERO_PAGE, it puts in a writable page if the vma flags permit, even when it's just a read fault (and its write_access arg is redundant).
> > I had noticed the difference in pte population between > force_page_cache_readahead and make_pages_present, but it seemed to me > that writing a function to walk the page tables and populate the > swapcache but not populate the ptes wasn't worth the effort.
I was about to agree with you, when you made the observation:
> Ah, another, more important difference: > > force_page_cache_readahead will not wait for the read to complete, > whereas get_user_pages() will be fully synchronous. > > I think I'd better come up with something else then,..
Yes, that's an interesting point. Maybe first put in what you have, to stop it from saying -EBADF on anon; then make it asynch later.
The asynch code: perhaps not worth doing for MADV_WILLNEED alone, but might prove useful for more general use when swapping in. Not really the same as Con's swap prefetch, but worth looking at that for reference. But I guess this becomes a much bigger issue than you were intending to get into here.
Hugh
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