From: Mark Kettenis Subject: Re: pgo_get() vs PG_BUSY To: Martin Pieuchot Cc: tech@openbsd.org, tb@openbsd.org Date: Tue, 17 Dec 2024 13:02:20 +0100 > Date: Tue, 17 Dec 2024 10:35:36 +0100 > From: Martin Pieuchot > > On 16/12/24(Mon) 21:07, Mark Kettenis wrote: > > > Date: Mon, 9 Dec 2024 10:49:56 +0100 > > > From: Martin Pieuchot > > > > > > On 07/12/24(Sat) 13:11, Mark Kettenis wrote: > > > > > Date: Tue, 3 Dec 2024 09:46:20 +0100 > > > > > From: Martin Pieuchot > > > > > > > > > > Diff below changes the behavior of the pgo_get() interface with PGO_LOCKED > > > > > and how the page currently fetched via uvm_fault_lower_io() is released. > > > > > > > > > > The pgo_get() w/ PGO_LOCKED implementations are always called with > > > > > `uobj->vmobjlock' locked and the lock is never released while manipulating > > > > > the pages. So there is not need to set the PG_BUSY flag on the pages. > > > > > This is what the uao_get() and uvn_get() chunks are about. > > > > > > > > > > The chunks in uvm_fault_lower_lookup() are simplifications related to > > > > > the fact that none of the pgo_get() set the PG_BUSY flags. > > > > > > > > > > The rest of the diff deals with `uobjpage', the physical page fetched > > > > > with pgo_get() w/ PGO_SYNCIO. Instead of releasing the pages in > > > > > multiple places release it as soon as we've got the associated lock in > > > > > uvm_fault_lower_io(). The reasoning is the same as above. > > > > > > > > > > This diff is the complicated part of the current parallelisation I'm > > > > > working on, so do not hesitate if you have any question. > > > > > > > > > > ok? > > > > > > > > This seems to leave behind a comment that states that pgo_get() will > > > > set the PG_BUSY flag in uvm_fault_lower_lookup(): > > > > > > > > /* > > > > * if center page is resident and not > > > > * PG_BUSY, then pgo_get made it PG_BUSY > > > > * for us and gave us a handle to it. > > > > * remember this page as "uobjpage." > > > > * (for later use). > > > > */ > > > > > > > > That needs to be fixed. > > > > > > Indeed, updated diff below with fixed comments. > > > > Great! > > > > > > [...] > > > > > @@ -1324,21 +1323,6 @@ uvm_fault_lower(struct uvm_faultinfo *ufi, struct uvm_faultctx *flt, > > > > > * out of memory resources? > > > > > */ > > > > > if (anon == NULL || pg == NULL) { > > > > > - /* > > > > > - * arg! must unbusy our page and fail or sleep. > > > > > - */ > > > > > - if (uobjpage != PGO_DONTCARE) { > > > > > - uvm_lock_pageq(); > > > > > - uvm_pageactivate(uobjpage); > > > > > - uvm_unlock_pageq(); > > > > > > > > Shouldn't we still activate the page in this case? > > > > > > Yes we should. We do it in uvm_fault_lower_io(). > > > > Hmm, but if we got the page through uvm_fault_lower_lookup() it > > doesn't happen since uvm_fault_lower_lookup() does an early continue > > for the center page. Or is that a corner cadse we don't really care > > about? > > I can't say if we don't really care or not. This activate disappeared > in NetBSD and I'd rather keep it for the moment. Thanks for spotting > it. > > Updated diff below. ok kettenis@ > Index: uvm/uvm_aobj.c > =================================================================== > RCS file: /cvs/src/sys/uvm/uvm_aobj.c,v > diff -u -p -r1.111 uvm_aobj.c > --- uvm/uvm_aobj.c 27 Nov 2024 10:41:38 -0000 1.111 > +++ uvm/uvm_aobj.c 17 Dec 2024 09:04:52 -0000 > @@ -1038,8 +1038,6 @@ uao_get(struct uvm_object *uobj, voff_t > /* > * useful page: plug it in our result array > */ > - atomic_setbits_int(&ptmp->pg_flags, PG_BUSY); > - UVM_PAGE_OWN(ptmp, "uao_get1"); > pps[lcv] = ptmp; > gotpages++; > } > Index: uvm/uvm_fault.c > =================================================================== > RCS file: /cvs/src/sys/uvm/uvm_fault.c,v > diff -u -p -r1.153 uvm_fault.c > --- uvm/uvm_fault.c 15 Dec 2024 11:02:59 -0000 1.153 > +++ uvm/uvm_fault.c 17 Dec 2024 09:31:57 -0000 > @@ -1140,14 +1140,13 @@ uvm_fault_lower_lookup( > pages[lcv] == PGO_DONTCARE) > continue; > > + KASSERT((pages[lcv]->pg_flags & PG_BUSY) == 0); > KASSERT((pages[lcv]->pg_flags & PG_RELEASED) == 0); > > /* > - * if center page is resident and not > - * PG_BUSY, then pgo_get made it PG_BUSY > - * for us and gave us a handle to it. > - * remember this page as "uobjpage." > - * (for later use). > + * if center page is resident and not PG_BUSY, then pgo_get > + * gave us a handle to it. > + * remember this page as "uobjpage." (for later use). > */ > if (lcv == flt->centeridx) { > uobjpage = pages[lcv]; > @@ -1155,8 +1154,13 @@ uvm_fault_lower_lookup( > } > > if (pmap_extract(ufi->orig_map->pmap, currva, &pa)) > - goto next; > + continue; > > + /* > + * calling pgo_get with PGO_LOCKED returns us pages which > + * are neither busy nor released, so we don't need to check > + * for this. we can just directly enter the pages. > + */ > if (pages[lcv]->wire_count == 0) { > uvm_lock_pageq(); > uvm_pageactivate(pages[lcv]); > @@ -1168,24 +1172,17 @@ uvm_fault_lower_lookup( > KASSERT(flt->wired == FALSE); > > /* > - * Since this page isn't the page that's > - * actually faulting, ignore pmap_enter() > - * failures; it's not critical that we > + * Since this page isn't the page that's actually faulting, > + * ignore pmap_enter() failures; it's not critical that we > * enter these right now. > + * NOTE: page can't be PG_WANTED or PG_RELEASED because we've > + * held the lock the whole time we've had the handle. > */ > (void) pmap_enter(ufi->orig_map->pmap, currva, > VM_PAGE_TO_PHYS(pages[lcv]) | flt->pa_flags, > flt->enter_prot & MASK(ufi->entry), PMAP_CANFAIL); > entered++; > > - /* > - * NOTE: page can't be PG_WANTED because > - * we've held the lock the whole time > - * we've had the handle. > - */ > -next: > - atomic_clearbits_int(&pages[lcv]->pg_flags, PG_BUSY); > - UVM_PAGE_OWN(pages[lcv], NULL); > } > if (entered > 0) > pmap_update(ufi->orig_map->pmap); > @@ -1273,6 +1270,11 @@ uvm_fault_lower(struct uvm_faultinfo *uf > if (uobjpage) { > /* update rusage counters */ > curproc->p_ru.ru_minflt++; > + if (uobjpage != PGO_DONTCARE) { > + uvm_lock_pageq(); > + uvm_pageactivate(uobjpage); > + uvm_unlock_pageq(); > + } > } else { > error = uvm_fault_lower_io(ufi, flt, &uobj, &uobjpage); > if (error != 0) > @@ -1330,21 +1332,6 @@ uvm_fault_lower(struct uvm_faultinfo *uf > * out of memory resources? > */ > if (anon == NULL || pg == NULL) { > - /* > - * arg! must unbusy our page and fail or sleep. > - */ > - if (uobjpage != PGO_DONTCARE) { > - uvm_lock_pageq(); > - uvm_pageactivate(uobjpage); > - uvm_unlock_pageq(); > - > - if (uobjpage->pg_flags & PG_WANTED) > - wakeup(uobjpage); > - atomic_clearbits_int(&uobjpage->pg_flags, > - PG_BUSY|PG_WANTED); > - UVM_PAGE_OWN(uobjpage, NULL); > - } > - > /* unlock and fail ... */ > uvmfault_unlockall(ufi, amap, uobj); > if (anon == NULL) > @@ -1395,16 +1382,6 @@ uvm_fault_lower(struct uvm_faultinfo *uf > pmap_page_protect(uobjpage, PROT_NONE); > } > #endif > - > - /* dispose of uobjpage. drop handle to uobj as well. */ > - if (uobjpage->pg_flags & PG_WANTED) > - wakeup(uobjpage); > - atomic_clearbits_int(&uobjpage->pg_flags, > - PG_BUSY|PG_WANTED); > - UVM_PAGE_OWN(uobjpage, NULL); > - uvm_lock_pageq(); > - uvm_pageactivate(uobjpage); > - uvm_unlock_pageq(); > /* done with copied uobjpage. */ > rw_exit(uobj->vmobjlock); > uobj = NULL; > @@ -1586,15 +1563,13 @@ uvm_fault_lower_io( > locked = FALSE; > } > > - /* didn't get the lock? release the page and retry. */ > - if (locked == FALSE && pg != PGO_DONTCARE) { > + /* release the page now, still holding object lock */ > + if (pg != PGO_DONTCARE) { > uvm_lock_pageq(); > - /* make sure it is in queues */ > uvm_pageactivate(pg); > uvm_unlock_pageq(); > > if (pg->pg_flags & PG_WANTED) > - /* still holding object lock */ > wakeup(pg); > atomic_clearbits_int(&pg->pg_flags, PG_BUSY|PG_WANTED); > UVM_PAGE_OWN(pg, NULL); > @@ -1607,7 +1582,8 @@ uvm_fault_lower_io( > } > > /* > - * we have the data in pg which is PG_BUSY > + * we have the data in pg. we are holding object lock (so the page > + * can't be released on us). > */ > *ruobj = uobj; > *ruobjpage = pg; > Index: uvm/uvm_vnode.c > =================================================================== > RCS file: /cvs/src/sys/uvm/uvm_vnode.c,v > diff -u -p -r1.135 uvm_vnode.c > --- uvm/uvm_vnode.c 27 Nov 2024 10:41:38 -0000 1.135 > +++ uvm/uvm_vnode.c 17 Dec 2024 09:04:52 -0000 > @@ -992,8 +992,6 @@ uvn_get(struct uvm_object *uobj, voff_t > * useful page: busy it and plug it in our > * result array > */ > - atomic_setbits_int(&ptmp->pg_flags, PG_BUSY); > - UVM_PAGE_OWN(ptmp, "uvn_get1"); > pps[lcv] = ptmp; > gotpages++; > > > >