> Hi List,
>
> I have tried this on kernelnewbies
> <http://article.gmane.org/gmane.linux.kernel.kernelnewbies/23166> but it seems
> I am not good at explaining well. trying here again.
>
> Question:
> Is there a limitation (some kind of caveat) when direct IO pages are added to
> a bio (multiple pages per bio) and if the total length or offset of some of
> the vectors is not aligned (PAGE_SIZE or sector size)?
>
> My scenario:
> 1. I have a filesystem in which I create bio for submission to a SCSI disk
> connected in a SAN via a FC switch.
> 2. This is a direct IO request where in multiple pages are being added in a bio
> (bio_map_user). Also, as the user buffer I get is unaligned, I am changing
> bio's first vector offset (as well as len, bi_size accordingly).
>
> What I observe is:
>
> 1. IO is performed perfectly (as expected) in case there are no SCSI failure.
> end_bio gets called as expected and the user app is happy.
> 2. In case I disconnect the SCSI device from switch, my end_bio function
> keeps on getting sub-completion bio (bi_size > 0 and uptodate cleared,
> error set to -5).
> 3. For capturing and ignoring sub-completion returns from block layer,
> using other kernel end_bio implementation as example I had a code like
>
> if(bio->bi_size > 0)
> return 1;
>
> as the starting lines of the end_bio routine. Thus, my user space goes off
> to disk sleep and kernel log keeps on spweing messages like
>
> ...
> Oct 17 21:56:07 mgs02 kernel: end_request: I/O error, dev sdl, sector 33639
> Oct 17 21:56:07 mgs02 kernel: sd 2:0:0:8: SCSI error: return code = 0x10000
> Oct 17 21:56:07 mgs02 kernel: end_request: I/O error, dev sdl, sector 33639
> Oct 17 21:56:07 mgs02 kernel: sd 2:0:0:8: SCSI error: return code = 0x10000
> Oct 17 21:56:07 mgs02 kernel: end_request: I/O error, dev sdl, sector 33639
> ...
>
> till i connect the switch back - even if that is one hour after disconnecting it.
>
> Best part - it works amazingly well in case I keep the offset of the first page
> aligned (somehow, at expense of corrupted data copy) - failure of SCSI disk
> (end_bio returns, bi_size=0, my code captures error) or no failure.
>
> I have tried this on 2.6.5 and 2.6.16.27 (I agree they are old, but that is
> what I have to work on :( ). It is a qlogic SCSI HBA. I have created the bio
> just like other kernel subsystems do.
>
> Only thing I am not sure is whether there is something special that needs to
> be done in case of direct IO which I am missing.
>
> biodoc.txt states that:
> "Note: Right now the only user of bios with more than one page is ll_rw_kio,
> which in turn means that only raw I/O uses it (direct i/o may not work right
> now)."
>
> Any idea why it doesn't work (though I am expecting that this doc is 2.6
> starting era and things might have changed a lot now).
>
> Any help would be highly appreciated. Apologies if my channel of approach is
> wrong and if this post is huge (couldn't keep it shorter than this).
>
> Shreyansh
>
>
> [PS: Is this situation something to do with request flags REQ_PC /
> REQ_BLOCK_PC?? While searching code, I found these somewhere connected to
> direct IO (bio_add_pc_page) and could find more about them]
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to
majordomo@vger.kernel.org
> More majordomo info at
http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at
http://www.tux.org/lkml/
