Messages in this thread | | | Date | Tue, 14 Apr 2009 22:39:54 +0900 | Subject | Re: [RFC][PATCH v3 1/6] mm: Don't unmap gup()ed page | From | KOSAKI Motohiro <> |
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>> >> @@ -547,7 +549,13 @@ int reuse_swap_page(struct page *page) >> >> SetPageDirty(page); >> >> } >> >> } >> >> - return count == 1; >> >> + >> >> + /* >> >> + * If we can re-use the swap page _and_ the end >> >> + * result has only one user (the mapping), then >> >> + * we reuse the whole page >> >> + */ >> >> + return count + page_count(page) == 2; >> >> } >> > >> > I guess this patch does work to close the read-side race, but I slightly don't >> > like using page_count for things like this. page_count can be temporarily >> > raised for reasons other than access through their user mapping. Swapcache, >> > page reclaim, LRU pagevecs, concurrent do_wp_page, etc. >> >> Yes, that's trade-off. >> your early decow also can misjudge and make unnecessary copy. > > Yes indeed it can. Although it would only ever do so in case of pages > that have had get_user_pages run against them previously, and not from > random interactions from any other parts of the kernel.
Agreed.
> I would be interested, using an anon vma field as you say for keeping > a gup count... it could potentially be used to avoid the extra copy. > But hmm, I don't have much time to go down that path so long as the > basic concept of my proposal is in question.
ok, I try to make it. thanks.
> + if (PageSwapCache(page) && > + page_count(page) != page_mapcount(page) + 2) { > + ret = SWAP_FAIL; > + goto out_unmap; > + } > > Now if another thread does a get_user_pages_fast after it passes this > check, it can take a gup reference to the page which is now about to > be unmapped. Then after it is unmapped, if a wp fault is caused on the > page, then it will not be reused and thus you lose data as explained > in your big comment.
Grrr, I lose. I've misunderstood get_user_pages_fast() also grab pte_lock. I must think it again.
I guess you dislike get_user_page_fast() grab pte_lock too, right? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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