Re: Oops in a driver while using SLUB as a SLAB allocator

[Christoph Lameter]

On Mon, 25 Jun 2007, Hugh Dickins wrote:

> > In many situations the page struct passed to flush_dcache_page is
> > simply used to calculate the virtual address. So its mostly harmless.
> > Trouble starts when page attributes like the mapping is used.
> 
> Mostly harmless indeed.  I don't understand why you insist on trying
> to complicate the situation.  flush_dcache_page is only expected to
> do something on pages mapped into userspace (correct me if I'm wrong
> there), it's expected to do nothing on kmalloc'ed pages.  It's
> been working that way for years, and will continue to work that way
> with slub, providing either page_mapping or flush_dcache_page checks
> PageSlab to avoid oopsing on page->mapping.

It is definitely intended to work. Otherwise we would not have code 
like this:

christoph@fly:~/linux-2.6$ find . -name "*.c" | xargs grep "flush_dcache_page"|grep virt
./drivers/scsi/scsi_tgt_if.c:   flush_dcache_page(virt_to_page(ev));
./drivers/scsi/scsi_tgt_if.c:           flush_dcache_page(virt_to_page(ev));


> 2.6.22-rc6 has page_mapping making that check: we could argue about
> which is the better site for it, there are good arguments both ways
> (page_mapping is the correct place, flush_dcache_page is the more
> efficient place), I suggest we leave it as is.

Ok. I think your patch is fine as a quick fix for 2.6.22. I am a bit 
uneasy with that given that its in such a broadly used function while its 
only use is to enable flush_dcache_page to work. But we need the general 
issue taken care of after 2.6.22.

> > A kmalloc slab object (even 64 byte) may be crossing a page boundary 
> > with a ARCH_KMALLOC_MINALIGN of 4 or 8. So I think that 
> > flush_dcache_range *must* be used rather than flush_dcache_page. 
> 
> Why????  All we require of flush_dcache_page is that it not oops on
> the first page in the range: we don't need to change over to
> flush_dcache_range for that.

As explained about: There are corner cases in which it does not work. You 
seem to assume that flush_dcache_page can become a no op. That may not be 
true on platforms that need explicit cache flushing for a DMA engine to 
access a data structure. The above listed use suggests that the caller 
expects flushing to occur correctly.



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