On Wed, 2015-07-29 at 16:22 -0400, Murali Karicheri wrote: > Eric, > > On 07/29/2015 12:31 PM, Eric Dumazet wrote: > > On Wed, 2015-07-29 at 11:10 -0400, WingMan Kwok wrote: > >> This patch makes the function __netdev_alloc_frag() non-static and > >> exports it so that drivers that need to specify additional flags, > >> such as __GFP_DMA, can use it. The currently exported function, > >> netdev_alloc_frag() doesn't allow passing in gfp_mask flags. > >> > >> Signed-off-by: WingMan Kwok <w-kw...@ti.com> > >> Signed-off-by: Reece R. Pollack <x0183...@ti.com> > >> --- > >> include/linux/skbuff.h | 1 + > >> net/core/skbuff.c | 3 ++- > >> 2 files changed, 3 insertions(+), 1 deletion(-) > > > > You can not do this. > > > > __napi_alloc_frag() uses __alloc_page_frag() using a per cpu reserve. > > > Thanks for your response. > > I assume you mean to say __netdev_alloc_frag() which is what the patch > affects. Right? > > > This per cpu reserve would be shared by regular GFP_ATOMIC and your > > __GFP_DMA allocations. > > I am trying to understand the issue here. Is there any issue in sharing > this per CPU reserve between DMA and ATOMIC allocations. Without this > flag, the assumption is this function can return memory which is not > DMA-able and this flag assures it is allocated from DMA zone.
First caller __netdev_alloc_frag() uses GFP_ATOMIC. A big page (32 KB) is allocated and stored into cache. Part of it given to caller. (like 1536 bytes or so) Then your driver calls with __GFP_DMA. We find a prior page on percpu cache with enough room in it to allocate a fragment. Your driver getd a fragment from the prior GFP_ATOMIC allocation, with no DMA guarantee. Therefore, your patch is not working in all cases. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
