2014-04-17 4:44 GMT+09:00 Andrew Morton <[email protected]>: > On Tue, 15 Apr 2014 22:08:45 +0900 Akinobu Mita <[email protected]> > wrote: > >> Calling dma_alloc_coherent() with __GFP_ZERO must return zeroed memory. >> >> But when the contiguous memory allocator (CMA) is enabled on x86 and >> the memory region is allocated by dma_alloc_from_contiguous(), it >> doesn't return zeroed memory. Because dma_generic_alloc_coherent() >> forgot to fill the memory region with zero if it was allocated by >> dma_alloc_from_contiguous() >> >> Most implementations of dma_alloc_coherent() return zeroed memory >> regardless of whether __GFP_ZERO is specified. So this fixes it by >> unconditionally zeroing the allocated memory region. >> >> Alternatively, we could fix dma_alloc_from_contiguous() to return >> zeroed out memory and remove memset() from all caller of it. But we >> can't simply remove the memset on arm because __dma_clear_buffer() is >> used there for ensuring cache flushing and it is used in many places. >> Of course we can do redundant memset in dma_alloc_from_contiguous(), >> but I think this patch is less impact for fixing this problem. > > But this patch does a duplicated memset if the page was allocated by > alloc_pages_node()?
You're right. Clearing __GFP_ZERO bit in gfp flags before allocating by alloc_pages_node() can fix this duplicated memset. > Would it not be better to pass the gfp_t to dma_alloc_from_contiguous() > and have it implement __GFP_ZERO? That will fix thsi inefficiency, > will be symmetrical with the other underlying allocators and should > permit the appropriate fixups in arm? Sounds good. If it also handles __GFP_WAIT, We can remove __GFP_WAIT check which is almost always required before calling dma_alloc_from_contiguous(). >> --- a/arch/x86/kernel/pci-dma.c >> +++ b/arch/x86/kernel/pci-dma.c >> @@ -97,7 +97,6 @@ void *dma_generic_alloc_coherent(struct device *dev, >> size_t size, >> >> dma_mask = dma_alloc_coherent_mask(dev, flag); >> >> - flag |= __GFP_ZERO; I'll soon prepare a follow-up patch to clear __GFP_ZERO like + flag &= ~__GFP_ZERO >> again: >> page = NULL; >> /* CMA can be used only in the context which permits sleeping */ >> @@ -120,7 +119,7 @@ again: >> >> return NULL; >> } >> - >> + memset(page_address(page), 0, size); >> *dma_addr = addr; >> return page_address(page); >> } >> -- >> 1.8.3.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
