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Date: Fri, 16 May 2014 11:32:39 +0100 From: Wei Liu <wei.liu2@...rix.com> To: Stefan Bader <stefan.bader@...onical.com> CC: Wei Liu <wei.liu2@...rix.com>, <xen-devel@...ts.xenproject.org>, Ian Campbell <Ian.Campbell@...rix.com>, Zoltan Kiss <zoltan.kiss@...rix.com>, netdev <netdev@...r.kernel.org> Subject: Re: [Xen-devel] xen-netfront possibly rides the rocket too often On Fri, May 16, 2014 at 12:17:48PM +0200, Stefan Bader wrote: > On 16.05.2014 12:09, Stefan Bader wrote: > > On 16.05.2014 11:48, Wei Liu wrote: > >> On Thu, May 15, 2014 at 02:14:00PM +0200, Stefan Bader wrote: > >> [...] > >>>> Wei. > >>>> > >>> Reading more of the code I would agree. The definition of MAX_SKB_FRAGS (at > >>> least now with compound pages) cannot be used in any way to derive the number of > >>> 4k slots a transfer will require. > >>> > >>> Zoltan already commented on worst cases. Not sure it would get as bad as that or > >>> "just" 16*4k frags all in the middle of compound pages. That would then end in > >>> around 33 or 34 slots, depending on the header. > >>> > >>> Zoltan wrote: > >>>> I think the worst case scenario is when every frag and the linear buffer contains 2 bytes, > >>>> which are overlapping a page boundary (that's (17+1)*2=36 so far), plus 15 of > >>> them have a 4k > >>>> page in the middle of them, so, a 1+4096+1 byte buffer can span over 3 page. > >>>> That's 51 individual pages. > >>> > >>> I cannot claim to really know what to expect worst case. Somewhat I was thinking > >>> of a > >>> worst case of (16+1)*2, which would be inconvenient enough. > >>> > >>> So without knowing exactly how to do it, but as Ian said it sounds best to come > >>> up with some sort of exception coalescing in cases the slot count goes over 18 > >>> and we know the data size is below 64K. > >>> > >> > >> I took a stab at it this morning and came up with this patch. Ran > >> redis-benchmark, it seemed to fix that for me -- only saw one "failed to > >> linearize skb" during > >> > >> redis-benchmark -h XXX -d 1000 -t lrange > >> > >> And before this change, a lot of "rides rocket" were triggered. > >> > >> Thought? > > > > It appears at least to me as something that nicely makes use of existing code. I > > was wondering about what could or could not be used. Trying to get ones head > > around the whole thing is kind of a lot to look at. > > > > The change at least looks straight forward enough. > > The only woe for me is that I am looking puzzled at the implementation of > skb_linearize(). Somehow the data_len element decides whether a skb can be > linearized and basically how much it tries to pull from the tail. It probably > makes sense ... just not to me with not deep experience here. > Data_len is the size of paged data (that's the size of data in frags). If it pulls everything from frags to linear area then SKB is linearized. Wei. > -Stefan > -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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