Re: How to map high memory for block io

[Jens Axboe <axboe@suse.de>]

On Fri, Jan 27 2006, Pierre Ossman wrote:
> Jens Axboe wrote:
> > On Fri, Jan 27 2006, Pierre Ossman wrote:
> >   
> >> Jens Axboe wrote:
> >>     
> >>> On Fri, Jan 27 2006, Pierre Ossman wrote:
> >>>   
> >>>       
> >>>> I'm having some problems getting high memory support to work smoothly in
> >>>> my driver. The documentation doesn't indicate what I might be doing
> >>>> wrong so I'll have to ask here.
> >>>>
> >>>> The problem seems to be that kmap & co maps a single page into kernel
> >>>> memory. So when I happen to cross page boundaries I start corrupting
> >>>> some unrelated parts of the kernel. I would prefer not having to
> >>>> consider page boundaries in an already messy PIO loop, so I've been
> >>>> trying to find either a routine to map an entire sg entry or some way to
> >>>> force the block layer to not give me stuff crossing pages.
> >>>>
> >>>> As you can guess I have not found anything that can do what I want, so
> >>>> some pointers would be nice.
> >>>>     
> >>>>         
> >>> Honestly, just don't bother if you are doing PIO anyways. Just tell the
> >>> block layer that you want io bounced for you instead.
> >>>
> >>>   
> >>>       
> >> This is the MMC layer so there is some separation between the block
> >> layer and the drivers. Also, the transfers won't necessarily be from the
> >> block layer so a generic solution is desired. I don't suppose there is
> >> some way of accessing the bounce buffer routines in a non-bio context?
> >>     
> >
> > Only the mapping routines are appropriate at that point, or things get
> > complicated. You could still do a two-page mapping, if you are careful
> > about using different KMAP_ types.
> >
> >   
> 
> That would still make things rather difficult since there is no way to
> get both maps into joining vaddrs. Is there no way to say "don't cross
> page boundaries"? Setting a segment size of PAGE_SIZE still causes
> problems when the offset isn't 0.

To be absolutely sure, you can just disallow multiple pages in a bio.
Ala:

static int my_merge_bvec_fn(request_queue_t *q, struct bio *bio,
                            struct bio_vec *bvec)
{
        return 1;
}

init_code()
{
        ...
        blk_queue_merge_bvec(q, my_merge_bvec_fn);
}

-- 
Jens Axboe

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