Date: Sat, 27 Aug 2016 21:31:00 +0000 (UTC) From: Alan Cox <alc@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-11@freebsd.org Subject: svn commit: r304922 - stable/11/sys/vm Message-ID: <201608272131.u7RLV0rS041535@repo.freebsd.org>
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Author: alc Date: Sat Aug 27 21:31:00 2016 New Revision: 304922 URL: https://svnweb.freebsd.org/changeset/base/304922 Log: MFC r303747,303982 Correct errors and clean up the comments on the active queue scan. Eliminate some unnecessary blank lines. Clean up the comments and code style in and around vm_pageout_cluster(). In particular, fix factual, grammatical, and spelling errors in various comments, and remove comments that are out of place in this function. Modified: stable/11/sys/vm/vm_pageout.c Directory Properties: stable/11/ (props changed) Modified: stable/11/sys/vm/vm_pageout.c ============================================================================== --- stable/11/sys/vm/vm_pageout.c Sat Aug 27 20:43:52 2016 (r304921) +++ stable/11/sys/vm/vm_pageout.c Sat Aug 27 21:31:00 2016 (r304922) @@ -355,41 +355,28 @@ vm_pageout_page_lock(vm_page_t m, vm_pag } /* - * vm_pageout_clean: - * - * Clean the page and remove it from the laundry. - * - * We set the busy bit to cause potential page faults on this page to - * block. Note the careful timing, however, the busy bit isn't set till - * late and we cannot do anything that will mess with the page. + * Scan for pages at adjacent offsets within the given page's object that are + * eligible for laundering, form a cluster of these pages and the given page, + * and launder that cluster. */ static int vm_pageout_cluster(vm_page_t m) { vm_object_t object; - vm_page_t mc[2*vm_pageout_page_count], pb, ps; - int pageout_count; - int ib, is, page_base; - vm_pindex_t pindex = m->pindex; + vm_page_t mc[2 * vm_pageout_page_count], p, pb, ps; + vm_pindex_t pindex; + int ib, is, page_base, pageout_count; - vm_page_lock_assert(m, MA_OWNED); + vm_page_assert_locked(m); object = m->object; VM_OBJECT_ASSERT_WLOCKED(object); + pindex = m->pindex; /* - * It doesn't cost us anything to pageout OBJT_DEFAULT or OBJT_SWAP - * with the new swapper, but we could have serious problems paging - * out other object types if there is insufficient memory. - * - * Unfortunately, checking free memory here is far too late, so the - * check has been moved up a procedural level. - */ - - /* - * Can't clean the page if it's busy or held. + * We can't clean the page if it is busy or held. */ vm_page_assert_unbusied(m); - KASSERT(m->hold_count == 0, ("vm_pageout_clean: page %p is held", m)); + KASSERT(m->hold_count == 0, ("page %p is held", m)); vm_page_unlock(m); mc[vm_pageout_page_count] = pb = ps = m; @@ -399,33 +386,23 @@ vm_pageout_cluster(vm_page_t m) is = 1; /* - * Scan object for clusterable pages. - * - * We can cluster ONLY if: ->> the page is NOT - * clean, wired, busy, held, or mapped into a - * buffer, and one of the following: - * 1) The page is inactive, or a seldom used - * active page. - * -or- - * 2) we force the issue. + * We can cluster only if the page is not clean, busy, or held, and + * the page is inactive. * * During heavy mmap/modification loads the pageout * daemon can really fragment the underlying file - * due to flushing pages out of order and not trying - * align the clusters (which leave sporatic out-of-order + * due to flushing pages out of order and not trying to + * align the clusters (which leaves sporadic out-of-order * holes). To solve this problem we do the reverse scan * first and attempt to align our cluster, then do a * forward scan if room remains. */ more: - while (ib && pageout_count < vm_pageout_page_count) { - vm_page_t p; - + while (ib != 0 && pageout_count < vm_pageout_page_count) { if (ib > pindex) { ib = 0; break; } - if ((p = vm_page_prev(pb)) == NULL || vm_page_busied(p)) { ib = 0; break; @@ -446,18 +423,16 @@ more: mc[--page_base] = pb = p; ++pageout_count; ++ib; + /* - * alignment boundary, stop here and switch directions. Do - * not clear ib. + * We are at an alignment boundary. Stop here, and switch + * directions. Do not clear ib. */ if ((pindex - (ib - 1)) % vm_pageout_page_count == 0) break; } - while (pageout_count < vm_pageout_page_count && pindex + is < object->size) { - vm_page_t p; - if ((p = vm_page_next(ps)) == NULL || vm_page_busied(p)) break; vm_page_test_dirty(p); @@ -477,15 +452,12 @@ more: /* * If we exhausted our forward scan, continue with the reverse scan - * when possible, even past a page boundary. This catches boundary - * conditions. + * when possible, even past an alignment boundary. This catches + * boundary conditions. */ - if (ib && pageout_count < vm_pageout_page_count) + if (ib != 0 && pageout_count < vm_pageout_page_count) goto more; - /* - * we allow reads during pageouts... - */ return (vm_pageout_flush(&mc[page_base], pageout_count, 0, 0, NULL, NULL)); } @@ -1216,15 +1188,13 @@ relock_queue: /* * Scan the active queue for pages that can be deactivated. Update * the per-page activity counter and use it to identify deactivation - * candidates. + * candidates. Held pages may be deactivated. */ for (m = TAILQ_FIRST(&pq->pq_pl), scanned = 0; m != NULL && (scanned < min_scan || (page_shortage > 0 && scanned < maxscan)); m = next, scanned++) { - KASSERT(m->queue == PQ_ACTIVE, ("vm_pageout_scan: page %p isn't active", m)); - next = TAILQ_NEXT(m, plinks.q); if ((m->flags & PG_MARKER) != 0) continue; @@ -1238,8 +1208,8 @@ relock_queue: } /* - * The count for pagedaemon pages is done after checking the - * page for eligibility... + * The count for page daemon pages is updated after checking + * the page for eligibility. */ PCPU_INC(cnt.v_pdpages); @@ -1253,12 +1223,17 @@ relock_queue: act_delta = 0; /* - * Unlocked object ref count check. Two races are possible. - * 1) The ref was transitioning to zero and we saw non-zero, - * the pmap bits will be checked unnecessarily. - * 2) The ref was transitioning to one and we saw zero. - * The page lock prevents a new reference to this page so - * we need not check the reference bits. + * Perform an unsynchronized object ref count check. While + * the page lock ensures that the page is not reallocated to + * another object, in particular, one with unmanaged mappings + * that cannot support pmap_ts_referenced(), two races are, + * nonetheless, possible: + * 1) The count was transitioning to zero, but we saw a non- + * zero value. pmap_ts_referenced() will return zero + * because the page is not mapped. + * 2) The count was transitioning to one, but we saw zero. + * This race delays the detection of a new reference. At + * worst, we will deactivate and reactivate the page. */ if (m->object->ref_count != 0) act_delta += pmap_ts_referenced(m);
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