Re: [PATCH] ata: pata_macio: Fix DMA table overflow

[Michael Ellerman <mpe@ellerman.id.au>]

Damien Le Moal <dlemoal@kernel.org> writes:
> On 8/19/24 19:17, Michael Ellerman wrote:
>> Kolbjørn and Jonáš reported that their 32-bit PowerMacs were crashing
>> in pata-macio since commit 09fe2bfa6b83 ("ata: pata_macio: Fix
>> max_segment_size with PAGE_SIZE == 64K").
>> 
>> For example:
>> 
>>   kernel BUG at drivers/ata/pata_macio.c:544!
>>   Oops: Exception in kernel mode, sig: 5 [#1]
>>   BE PAGE_SIZE=4K MMU=Hash SMP NR_CPUS=2 DEBUG_PAGEALLOC PowerMac
>>   ...
>>   NIP pata_macio_qc_prep+0xf4/0x190
>>   LR  pata_macio_qc_prep+0xfc/0x190
>>   Call Trace:
>>     0xc1421660 (unreliable)
>>     ata_qc_issue+0x14c/0x2d4
>>     __ata_scsi_queuecmd+0x200/0x53c
>>     ata_scsi_queuecmd+0x50/0xe0
>>     scsi_queue_rq+0x788/0xb1c
>>     __blk_mq_issue_directly+0x58/0xf4
>>     blk_mq_plug_issue_direct+0x8c/0x1b4
>>     blk_mq_flush_plug_list.part.0+0x584/0x5e0
>>     __blk_flush_plug+0xf8/0x194
>>     __submit_bio+0x1b8/0x2e0
>>     submit_bio_noacct_nocheck+0x230/0x304
>>     btrfs_work_helper+0x200/0x338
>>     process_one_work+0x1a8/0x338
>>     worker_thread+0x364/0x4c0
>>     kthread+0x100/0x104
>>     start_kernel_thread+0x10/0x14
>> 
>> That commit increased max_segment_size to 64KB, with the justification
>> that the SCSI core was already using that size when PAGE_SIZE == 64KB,
>> and that there was existing logic to split over-sized requests.
>> 
>> However with a sufficiently large request, the splitting logic causes
>> each sg to be split into two commands in the DMA table, leading to
>> overflow of the DMA table, triggering the BUG_ON().
>> 
>> With default settings the bug doesn't trigger, because the request size
>> is limited by max_sectors_kb == 1280, however max_sectors_kb can be
>> increased, and apparently some distros do that by default using udev
>> rules.
>> 
>> Fix the bug for 4KB kernels by reverting to the old max_segment_size.
>> 
>> For 64KB kernels the sg_tablesize needs to be halved, to allow for the
>> possibility that each sg will be split into two.
>> 
>> Fixes: 09fe2bfa6b83 ("ata: pata_macio: Fix max_segment_size with PAGE_SIZE == 64K")
>> Cc: stable@vger.kernel.org # v6.10+
>> Reported-by: Kolbjørn Barmen <linux-ppc@kolla.no>
>> Closes: https://lore.kernel.org/all/62d248bb-e97a-25d2-bcf2-9160c518cae5@kolla.no/
>> Reported-by: Jonáš Vidra <vidra@ufal.mff.cuni.cz>
>> Closes: https://lore.kernel.org/all/3b6441b8-06e6-45da-9e55-f92f2c86933e@ufal.mff.cuni.cz/
>> Tested-by: Kolbjørn Barmen <linux-ppc@kolla.no>
>> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
>> ---
>>  drivers/ata/pata_macio.c | 23 +++++++++++++++--------
>>  1 file changed, 15 insertions(+), 8 deletions(-)
>> 
>> diff --git a/drivers/ata/pata_macio.c b/drivers/ata/pata_macio.c
>> index 1b85e8bf4ef9..eaffa510de49 100644
>> --- a/drivers/ata/pata_macio.c
>> +++ b/drivers/ata/pata_macio.c
>> @@ -208,6 +208,19 @@ static const char* macio_ata_names[] = {
>>  /* Don't let a DMA segment go all the way to 64K */
>>  #define MAX_DBDMA_SEG		0xff00
>>  
>> +#ifdef CONFIG_PAGE_SIZE_64KB
>> +/*
>> + * The SCSI core requires the segment size to cover at least a page, so
>> + * for 64K page size kernels it must be at least 64K. However the
>> + * hardware can't handle 64K, so pata_macio_qc_prep() will split large
>> + * requests. To handle the split requests the tablesize must be halved.
>> + */
>> +#define MAX_SEGMENT_SIZE SZ_64K
>> +#define SG_TABLESIZE (MAX_DCMDS / 2)
>> +#else
>> +#define MAX_SEGMENT_SIZE MAX_DBDMA_SEG
>> +#define SG_TABLESIZE MAX_DCMDS
>> +#endif
>
> These names are rather generic and could clash with some core layer ditions. So
> maybe prefix the macro names with PATA_MACIO_ ?
> Also please tab-align the values to make this a little easier to read.
 
OK.

> Other than this, looks good to me.

OK thanks, will send a v2.

cheers