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Thu, 20 Feb 2020 12:54:44 -0800 (PST) Date: Thu, 20 Feb 2020 15:54:43 -0500 From: Peter Xu To: David Hildenbrand Subject: Re: [PATCH v1 10/13] migrate/ram: Handle RAM block resizes during postcopy Message-ID: <20200220205443.GD15253@xz-x1> References: <20200219161725.115218-1-david@redhat.com> <20200219161725.115218-11-david@redhat.com> MIME-Version: 1.0 In-Reply-To: <20200219161725.115218-11-david@redhat.com> X-MC-Unique: suWEc39cNgCiedfE5pr-4w-1 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 207.211.31.120 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Eduardo Habkost , Juan Quintela , "Michael S. Tsirkin" , Richard Henderson , qemu-devel@nongnu.org, "Dr . David Alan Gilbert" , Shannon Zhao , Igor Mammedov , Paolo Bonzini , Alex =?utf-8?Q?Benn=C3=A9e?= , Richard Henderson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Feb 19, 2020 at 05:17:22PM +0100, David Hildenbrand wrote: > Resizing while migrating is dangerous and does not work as expected. > The whole migration code works on the usable_length of ram blocks and doe= s > not expect this to change at random points in time. >=20 > In the case of postcopy, relying on used_length is racy as soon as the > guest is running. Also, when used_length changes we might leave the > uffd handler registered for some memory regions, reject valid pages > when migrating and fail when sending the recv bitmap to the source. >=20 > Resizing can be trigger *after* (but not during) a reset in > ACPI code by the guest > - hw/arm/virt-acpi-build.c:acpi_ram_update() > - hw/i386/acpi-build.c:acpi_ram_update() >=20 > Let's remember the original used_length in a separate variable and > use it in relevant postcopy code. Make sure to update it when we resize > during precopy, when synchronizing the RAM block sizes with the source. >=20 > Cc: "Dr. David Alan Gilbert" > Cc: Juan Quintela > Cc: Eduardo Habkost > Cc: Paolo Bonzini > Cc: Igor Mammedov > Cc: "Michael S. Tsirkin" > Cc: Richard Henderson > Cc: Shannon Zhao > Cc: Alex Benn=C3=A9e > Cc: Peter Xu > Signed-off-by: David Hildenbrand > --- > include/exec/ramblock.h | 9 +++++++++ > migration/postcopy-ram.c | 15 ++++++++++++--- > migration/ram.c | 11 +++++++++-- > 3 files changed, 30 insertions(+), 5 deletions(-) >=20 > diff --git a/include/exec/ramblock.h b/include/exec/ramblock.h > index 07d50864d8..0e9e9b346b 100644 > --- a/include/exec/ramblock.h > +++ b/include/exec/ramblock.h > @@ -59,6 +59,15 @@ struct RAMBlock { > */ > unsigned long *clear_bmap; > uint8_t clear_bmap_shift; > + > + /* > + * RAM block used_length before the guest started running while post= copy > + * was active. Once the guest is running, used_length can change. Us= ed to > + * register/unregister uffd handlers and as the size of the recv bit= map. > + * Receiving any page beyond this length will bail out, as it could = not have > + * been valid on the source. > + */ > + ram_addr_t postcopy_length; > }; > #endif > #endif > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c > index a36402722b..c68caf4e42 100644 > --- a/migration/postcopy-ram.c > +++ b/migration/postcopy-ram.c > @@ -17,6 +17,7 @@ > */ > =20 > #include "qemu/osdep.h" > +#include "qemu/rcu.h" > #include "exec/target_page.h" > #include "migration.h" > #include "qemu-file.h" > @@ -31,6 +32,7 @@ > #include "qemu/error-report.h" > #include "trace.h" > #include "hw/boards.h" > +#include "exec/ramblock.h" > =20 > /* Arbitrary limit on size of each discard command, > * keeps them around ~200 bytes > @@ -456,6 +458,13 @@ static int init_range(RAMBlock *rb, void *opaque) > ram_addr_t length =3D qemu_ram_get_used_length(rb); > trace_postcopy_init_range(block_name, host_addr, offset, length); > =20 > + /* > + * Save the used_length before running the guest. In case we have to > + * resize RAM blocks when syncing RAM block sizes from the source du= ring > + * precopy, we'll update it manually via the ram block notifier. > + */ > + rb->postcopy_length =3D length; > + > /* > * We need the whole of RAM to be truly empty for postcopy, so thing= s > * like ROMs and any data tables built during init must be zero'd > @@ -478,7 +487,7 @@ static int cleanup_range(RAMBlock *rb, void *opaque) > const char *block_name =3D qemu_ram_get_idstr(rb); > void *host_addr =3D qemu_ram_get_host_addr(rb); > ram_addr_t offset =3D qemu_ram_get_offset(rb); > - ram_addr_t length =3D qemu_ram_get_used_length(rb); > + ram_addr_t length =3D rb->postcopy_length; > MigrationIncomingState *mis =3D opaque; > struct uffdio_range range_struct; > trace_postcopy_cleanup_range(block_name, host_addr, offset, length); > @@ -600,7 +609,7 @@ static int nhp_range(RAMBlock *rb, void *opaque) > const char *block_name =3D qemu_ram_get_idstr(rb); > void *host_addr =3D qemu_ram_get_host_addr(rb); > ram_addr_t offset =3D qemu_ram_get_offset(rb); > - ram_addr_t length =3D qemu_ram_get_used_length(rb); > + ram_addr_t length =3D rb->postcopy_length; > trace_postcopy_nhp_range(block_name, host_addr, offset, length); > =20 > /* > @@ -644,7 +653,7 @@ static int ram_block_enable_notify(RAMBlock *rb, void= *opaque) > struct uffdio_register reg_struct; > =20 > reg_struct.range.start =3D (uintptr_t)qemu_ram_get_host_addr(rb); > - reg_struct.range.len =3D qemu_ram_get_used_length(rb); > + reg_struct.range.len =3D rb->postcopy_length; > reg_struct.mode =3D UFFDIO_REGISTER_MODE_MISSING; > =20 > /* Now tell our userfault_fd that it's responsible for this area */ > diff --git a/migration/ram.c b/migration/ram.c > index ab1f5534cf..6d1dcb362c 100644 > --- a/migration/ram.c > +++ b/migration/ram.c > @@ -244,7 +244,7 @@ int64_t ramblock_recv_bitmap_send(QEMUFile *file, > return -1; > } > =20 > - nbits =3D block->used_length >> TARGET_PAGE_BITS; > + nbits =3D block->postcopy_length >> TARGET_PAGE_BITS; > =20 > /* > * Make sure the tmp bitmap buffer is big enough, e.g., on 32bit > @@ -3159,7 +3159,13 @@ static int ram_load_postcopy(QEMUFile *f) > break; > } > =20 > - if (!offset_in_ramblock(block, addr)) { > + /* > + * Relying on used_length is racy and can result in false po= sitives. > + * We might place pages beyond used_length in case RAM was s= hrunk > + * while in postcopy, which is fine - trying to place via > + * UFFDIO_COPY/UFFDIO_ZEROPAGE will never segfault. > + */ > + if (!block->host || addr >=3D block->postcopy_length) { > error_report("Illegal RAM offset " RAM_ADDR_FMT, addr); > ret =3D -EINVAL; > break; > @@ -3744,6 +3750,7 @@ static void ram_mig_ram_block_resized(RAMBlockNotif= ier *n, void *host, > rb->idstr); > } > } > + rb->postcopy_length =3D new_size; With this change, postcopy_length will be the same as used_length? I thought you wanted to cache that value when starting the postcopy phase so postcopy_length should be constant after set once. Did I misunderstood? Thanks, > return; > } > =20 > --=20 > 2.24.1 >=20 --=20 Peter Xu