Messages in this thread | | | Date | Wed, 16 May 2007 23:09:11 +0530 | From | "Satyam Sharma" <> | Subject | Re: Pagecache zeroing: zero_user_segment, zero_user_segments and zero_user |
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On 5/16/07, Andrew Morton <akpm@linux-foundation.org> wrote: > > > > 2. It has a useless parameter specifying the kmap slot. > > The functions above default to KM_USER0 which is also always used when > > zero_user_page was called except in one single case. We open code that > > single case to draw attention to the spot. > > > > Dunno. fwiw, we decided to _not_ embed KM_USER0 in the callee: we have had > some pretty ghastly bugs in the past due to misuse of kmap slots so the > idea was to shove the decision into the caller's face, make them think > about what they're doing
Christoph hasn't really _replaced_ any !KM_USER0 case with KM_USER0 in this patch. AFAIR, we had originally decided to not embed KM_USER0 in zero_user_page because of this complaint from Anton Altaparmakov: http://lkml.org/lkml/2007/4/10/62
But Christoph's patch here seems to have left that fs/ntfs/aops.c case alone, and used the zero_xxx helpers only for KM_USER0 cases, so we should probably be safe.
> > +static inline void zero_user_segments(struct page *page, > > + unsigned start1, unsigned end1, > > + unsigned start2, unsigned end2) > > +{ > > + void *kaddr = kmap_atomic(page, KM_USER0); > > + > > + BUG_ON(end1 > PAGE_SIZE || > > + end2 > PAGE_SIZE); > > + > > + if (end1 > start1) > > + memset(kaddr + start1, 0, end1 - start1); > > + > > + if (end2 > start2) > > + memset(kaddr + start2, 0, end2 - start2); > > + > > + flush_dcache_page(page); > > + kunmap_atomic(kaddr, KM_USER0); > > +} > > For some reason we've always done the flush_dcache_page() while holding the > kmap. There's no reason for doing it this way and it just serves to worsen > scheduling latency a tiny bit.
Wonder if any arch requires a valid mapping when doing the flush?
(reading Documentation/cachetlb.txt)
Documentation/cachetlb.txt is silent on whether flush_dcache_page() really must be called before a kunmap(), but mentions that flush_kernel_dcache_page() _must_ be called _before_ kunmap. And I saw that parisc implements flush_dcache_page() using flush_kernel_dcache_page itself (but then, we've not implemented a meaningful kmap for parisc in the first place), so we've _just_ managed to be fine here ...
Still, flush_dcache_page() before kunmap'ing is an old practice, would be wise to get this whetted by linux-arch first?
Satyam - 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/
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