Messages in this thread | | | Date | Mon, 17 Oct 2005 19:53:27 +0200 | From | Jens Axboe <> | Subject | Re: A problem about DIRECT IO on ext3 |
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On Mon, Oct 17 2005, Badari Pulavarty wrote: > On Mon, 2005-10-17 at 11:51 +0200, Jens Axboe wrote: > > On Mon, Oct 17 2005, li nux wrote: > > > > > > > > > --- Jens Axboe <axboe@suse.de> wrote: > > > > > > > On Mon, Oct 17 2005, Grzegorz Kulewski wrote: > > > > > On Mon, 17 Oct 2005, Jens Axboe wrote: > > > > > >>how to correct this problem ? > > > > > > > > > > > >See your buffer address, it's not aligned. You > > > > need to align that as > > > > > >well. This is needed because the hardware will > > > > dma directly to the user > > > > > >buffer, and to be on the safe side we require the > > > > same alignment as the > > > > > >block layer will normally generate for file > > > > system io. > > > > > > > > > > > >So in short, just align your read buffer to the > > > > same as your block size > > > > > >and you will be fine. Example: > > > > > > > > > > > >#define BS (4096) > > > > > >#define MASK (BS - 1) > > > > > >#define ALIGN(buf) (((unsigned long) (buf) + > > > > MASK) & ~(MASK)) > > > > > > > > > > > >char *ptr = malloc(BS + MASK); > > > > > >char *buf = (char *) ALIGN(ptr); > > > > > > > > > > > >read(fd, buf, BS); > > > > > > > > > > Shouldn't one use posix_memalign(3) for that? > > > > > > > > Dunno if one 'should', one 'can' if one wants to. I > > > > prefer to do it > > > > manually so I don't have to jump through #define > > > > hoops to get at it > > > > (which, btw, still doesn't expose it on this > > > > machine). > > > > > > > > -- > > > > Jens Axboe > > > > > > Thanx a lot Jens :-) > > > Its working now. > > > I did not have to make these adjustments on 2.6 > > > Is looks to be having more relaxation. > > > > 2.6 does have the option of checking the hardware dma requirement > > seperately, but for this path you should run into the same restrictions. > > Perhaps you just got lucky when testing 2.6? > > 2.6 also has the same restriction. But, if the "filesystem > blocksize alignment" (soft block size) fails, we try to see > if its aligned with hard sector size (512). If so, we can do the IO. > > 2.4 fails if the offset or buffer is NOT filesystem blocksize > aligned. Period.
I'm aware of that, however this particular case was about the buffer alignment (which was 32-bytes in the strace). And that should not work for 2.6 either.
The block-size alignment is really a separate property of correctness.
> BTW, posix_memalign() or valloc() should be safe.
Certainly.
-- Jens Axboe
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