Re: [PATCH v8 2/8] xen: mapcache: Unmap first entries in buckets

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Mon, Jul 01, 2024 at 04:34:53PM +0200, Edgar E. Iglesias wrote:
> On Mon, Jul 1, 2024 at 4:30 PM Edgar E. Iglesias <edgar.iglesias@xxxxxxxxx>
> wrote:
> > On Mon, Jul 1, 2024 at 3:58 PM Edgar E. Iglesias <edgar.iglesias@xxxxxxxxx>
> > wrote:
> >> Any chance you could try to get a backtrace from QEMU when it failed?

Here it is:


#3  0x00007fa8762f4472 in __GI_abort () at ./stdlib/abort.c:79
        save_stage = 1
        act = {__sigaction_handler = {sa_handler = 0x20, sa_sigaction = 0x20}, 
sa_mask = {__val = {94603440166168, 18446744073709551615, 94603406369640, 0, 0, 
94603406346720, 94603440166168, 140361486774256, 0, 140361486773376, 
94603401285536, 140361496232688, 94603440166096, 140361486773456, 
94603401289576, 140360849280256}}, sa_flags = -1804462896, sa_restorer = 
0x748f2d40}
#4  0x0000560a92230f0d in qemu_get_ram_block (addr=18446744073709551615) at 
../system/physmem.c:801
        block = 0x0
#5  0x0000560a922350ab in qemu_ram_block_from_host (ptr=0x7fa84e8fcd00, 
round_offset=false, offset=0x7fa8748f2de8) at ../system/physmem.c:2280
        ram_addr = 18446744073709551615
        _rcu_read_auto = 0x1
        block = 0x0
        host = 0x7fa84e8fcd00 ""
        _rcu_read_auto = 0x7fa8751f8288
#6  0x0000560a92229669 in memory_region_from_host (ptr=0x7fa84e8fcd00, 
offset=0x7fa8748f2de8) at ../system/memory.c:2440
        block = 0x0
#7  0x0000560a92237418 in address_space_unmap (as=0x560a94b05a20, 
buffer=0x7fa84e8fcd00, len=32768, is_write=true, access_len=32768) at 
../system/physmem.c:3246
        mr = 0x0
        addr1 = 0
        __PRETTY_FUNCTION__ = "address_space_unmap"
#8  0x0000560a91fd6cd3 in dma_memory_unmap (as=0x560a94b05a20, 
buffer=0x7fa84e8fcd00, len=32768, dir=DMA_DIRECTION_FROM_DEVICE, 
access_len=32768) at /root/build/qemu/include/sysemu/dma.h:236
#9  0x0000560a91fd763d in dma_blk_unmap (dbs=0x560a94d87400) at 
../system/dma-helpers.c:93
        i = 1
#10 0x0000560a91fd76e6 in dma_complete (dbs=0x560a94d87400, ret=0) at 
../system/dma-helpers.c:105
        __PRETTY_FUNCTION__ = "dma_complete"
#11 0x0000560a91fd781c in dma_blk_cb (opaque=0x560a94d87400, ret=0) at 
../system/dma-helpers.c:129
        dbs = 0x560a94d87400
        ctx = 0x560a9448da90
        cur_addr = 0
        cur_len = 0
        mem = 0x0
        __PRETTY_FUNCTION__ = "dma_blk_cb"
#12 0x0000560a9232e974 in blk_aio_complete (acb=0x560a9448d5f0) at 
../block/block-backend.c:1555
#13 0x0000560a9232ebd1 in blk_aio_read_entry (opaque=0x560a9448d5f0) at 
../block/block-backend.c:1610
        acb = 0x560a9448d5f0
        rwco = 0x560a9448d618
        qiov = 0x560a94d87460
        __PRETTY_FUNCTION__ = "blk_aio_read_entry"

> > One more thing, regarding this specific patch. I don't think we should
> > clear the
> > entire entry, the next field should be kept, otherwise we'll disconnect
> > following
> > mappings that will never be found again. IIUC, this could very well be
> > causing the problem you see.
> >
> > Does the following make sense?
> >
> And here without double-freeing entry->valid_mapping:
>
> diff --git a/hw/xen/xen-mapcache.c b/hw/xen/xen-mapcache.c
> index 5f23b0adbe..667807b3b6 100644
> --- a/hw/xen/xen-mapcache.c
> +++ b/hw/xen/xen-mapcache.c
> @@ -597,7 +597,13 @@ static void
> xen_invalidate_map_cache_entry_unlocked(MapCache *mc,
>          pentry->next = entry->next;
>          g_free(entry);
>      } else {
> -        memset(entry, 0, sizeof *entry);
> +        /* Invalidate mapping.  */
> +        entry->paddr_index = 0;
> +        entry->vaddr_base = NULL;
> +        entry->size = 0;
> +        entry->valid_mapping = NULL;
> +        entry->flags = 0;
> +        /* Keep entry->next pointing to the rest of the list.  */
>      }
>  }

I've tried this patch, and that fix the issue I've seen. I'll run more
tests on it, just in case, but there's no reason that would break
something else.

Cheers,


--

Anthony Perard | Vates XCP-ng Developer

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