Date: Thu, 14 Jun 2018 18:07:52 -0500 From: Alan Cox <alc@rice.edu> To: Steven Hartland <steven.hartland@multiplay.co.uk> Cc: Konstantin Belousov <kib@FreeBSD.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r335171 - head/sys/vm Message-ID: <F220E356-D55E-48B9-9AF0-ABBF41A74FC7@rice.edu> In-Reply-To: <c241bd08-eb94-152e-c1f1-d77dc6987908@multiplay.co.uk> References: <201806141941.w5EJf2qa069373@repo.freebsd.org> <c241bd08-eb94-152e-c1f1-d77dc6987908@multiplay.co.uk>
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> On Jun 14, 2018, at 5:54 PM, Steven Hartland = <steven.hartland@multiplay.co.uk> wrote: >=20 > Out of interest, how would this exhibit itself? >=20 A panic in vm_page_insert_after(). > On 14/06/2018 20:41, Konstantin Belousov wrote: >> Author: kib >> Date: Thu Jun 14 19:41:02 2018 >> New Revision: 335171 >> URL: https://svnweb.freebsd.org/changeset/base/335171 = <https://svnweb.freebsd.org/changeset/base/335171>; >>=20 >> Log: >> Handle the race between fork/vm_object_split() and faults. >> =20 >> If fault started before vmspace_fork() locked the map, and then = during >> fork, vm_map_copy_entry()->vm_object_split() is executed, it is >> possible that the fault instantiate the page into the original = object >> when the page was already copied into the new object (see >> vm_map_split() for the orig/new objects terminology). This can = happen >> if split found a busy page (e.g. from the fault) and slept dropping >> the objects lock, which allows the swap pager to instantiate >> read-behind pages for the fault. Then the restart of the scan can = see >> a page in the scanned range, where it was already copied to the = upper >> object. >> =20 >> Fix it by instantiating the read-ahead pages before >> swap_pager_getpages() method drops the lock to allocate pbuf. The >> object scan would see the whole range prefilled with the busy pages >> and not proceed the range. >> =20 >> Note that vm_fault rechecks the map generation count after the = object >> unlock, so that it restarts the handling if raced with split, and >> re-lookups the right page from the upper object. >> =20 >> In collaboration with: alc >> Tested by: pho >> Sponsored by: The FreeBSD Foundation >> MFC after: 1 week >>=20 >> Modified: >> head/sys/vm/swap_pager.c >>=20 >> Modified: head/sys/vm/swap_pager.c >> = =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D >> --- head/sys/vm/swap_pager.c Thu Jun 14 19:01:40 2018 = (r335170) >> +++ head/sys/vm/swap_pager.c Thu Jun 14 19:41:02 2018 = (r335171) >> @@ -1096,21 +1096,24 @@ swap_pager_getpages(vm_object_t object, = vm_page_t *ma, >> int *rahead) >> { >> struct buf *bp; >> - vm_page_t mpred, msucc, p; >> + vm_page_t bm, mpred, msucc, p; >> vm_pindex_t pindex; >> daddr_t blk; >> - int i, j, maxahead, maxbehind, reqcount, shift; >> + int i, maxahead, maxbehind, reqcount; >> =20 >> reqcount =3D count; >> =20 >> - VM_OBJECT_WUNLOCK(object); >> - bp =3D getpbuf(&nsw_rcount); >> - VM_OBJECT_WLOCK(object); >> - >> - if (!swap_pager_haspage(object, ma[0]->pindex, &maxbehind, = &maxahead)) { >> - relpbuf(bp, &nsw_rcount); >> + /* >> + * Determine the final number of read-behind pages and >> + * allocate them BEFORE releasing the object lock. Otherwise, >> + * there can be a problematic race with vm_object_split(). >> + * Specifically, vm_object_split() might first transfer pages >> + * that precede ma[0] in the current object to a new object, >> + * and then this function incorrectly recreates those pages as >> + * read-behind pages in the current object. >> + */ >> + if (!swap_pager_haspage(object, ma[0]->pindex, &maxbehind, = &maxahead)) >> return (VM_PAGER_FAIL); >> - } >> =20 >> /* >> * Clip the readahead and readbehind ranges to exclude resident = pages. >> @@ -1132,35 +1135,31 @@ swap_pager_getpages(vm_object_t object, = vm_page_t *ma, >> *rbehind =3D pindex - mpred->pindex - 1; >> } >> =20 >> + bm =3D ma[0]; >> + for (i =3D 0; i < count; i++) >> + ma[i]->oflags |=3D VPO_SWAPINPROG; >> + >> /* >> * Allocate readahead and readbehind pages. >> */ >> - shift =3D rbehind !=3D NULL ? *rbehind : 0; >> - if (shift !=3D 0) { >> - for (i =3D 1; i <=3D shift; i++) { >> + if (rbehind !=3D NULL) { >> + for (i =3D 1; i <=3D *rbehind; i++) { >> p =3D vm_page_alloc(object, ma[0]->pindex - i, >> VM_ALLOC_NORMAL); >> - if (p =3D=3D NULL) { >> - /* Shift allocated pages to the left. */ >> - for (j =3D 0; j < i - 1; j++) >> - bp->b_pages[j] =3D >> - bp->b_pages[j + shift - i + = 1]; >> + if (p =3D=3D NULL) >> break; >> - } >> - bp->b_pages[shift - i] =3D p; >> + p->oflags |=3D VPO_SWAPINPROG; >> + bm =3D p; >> } >> - shift =3D i - 1; >> - *rbehind =3D shift; >> + *rbehind =3D i - 1; >> } >> - for (i =3D 0; i < reqcount; i++) >> - bp->b_pages[i + shift] =3D ma[i]; >> if (rahead !=3D NULL) { >> for (i =3D 0; i < *rahead; i++) { >> p =3D vm_page_alloc(object, >> ma[reqcount - 1]->pindex + i + 1, = VM_ALLOC_NORMAL); >> if (p =3D=3D NULL) >> break; >> - bp->b_pages[shift + reqcount + i] =3D p; >> + p->oflags |=3D VPO_SWAPINPROG; >> } >> *rahead =3D i; >> } >> @@ -1171,15 +1170,18 @@ swap_pager_getpages(vm_object_t object, = vm_page_t *ma, >> =20 >> vm_object_pip_add(object, count); >> =20 >> - for (i =3D 0; i < count; i++) >> - bp->b_pages[i]->oflags |=3D VPO_SWAPINPROG; >> - >> - pindex =3D bp->b_pages[0]->pindex; >> + pindex =3D bm->pindex; >> blk =3D swp_pager_meta_ctl(object, pindex, 0); >> KASSERT(blk !=3D SWAPBLK_NONE, >> ("no swap blocking containing %p(%jx)", object, = (uintmax_t)pindex)); >> =20 >> VM_OBJECT_WUNLOCK(object); >> + bp =3D getpbuf(&nsw_rcount); >> + /* Pages cannot leave the object while busy. */ >> + for (i =3D 0, p =3D bm; i < count; i++, p =3D TAILQ_NEXT(p, = listq)) { >> + MPASS(p->pindex =3D=3D bm->pindex + i); >> + bp->b_pages[i] =3D p; >> + } >> =20 >> bp->b_flags |=3D B_PAGING; >> bp->b_iocmd =3D BIO_READ; >>=20 >=20
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