On Wed, 2016-11-23 at 13:55 -0500, Laurence Oberman wrote: > > ----- Original Message ----- > > From: "Eyal Ben David" <bdeyal@xxxxxxxxx> > > To: "Ewan D. Milne" <emilne@xxxxxxxxxx> > > Cc: "Johannes Thumshirn" <jthumshirn@xxxxxxx>, dgilbert@xxxxxxxxxxxx, "Laurence Oberman" <loberman@xxxxxxxxxx>, > > linux-scsi@xxxxxxxxxxxxxxx > > Sent: Tuesday, November 22, 2016 3:55:44 PM > > Subject: Re: SG does not ignore dxferp (direct io + mmap) > > > > On Tue, Nov 22, 2016 at 8:30 PM, Ewan D. Milne <emilne@xxxxxxxxxx> wrote: > > > > > > I see the behavior (zero byte) on the 4.4.34, 4.5.7, 4.6.7, and 4.7.10 > > > -stable kernels. But not (of course) on 4.8.10 -stable. > > > > > > It doesn't look like the sg driver, might be something in the mmap code? > > > > > > A kernel guy colleague suggested to look at copy_from_user / copy_to_user > > code. > > It was changed in 4.8 > > > > It was OK with 3.13 (Ubuntu 14.04) but from some kernel (prior or equal to > > 4.4) > > until 4.7 we see the bug. It was somehow fixed at 4.8. > > > > In order to fully understand what happened, there are two changes to find. > > They might not even be related. > > > > Thanks! > > Eyal > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-scsi" in > > the body of a message to majordomo@xxxxxxxxxxxxxxx > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > > So 4.7.9 fails and 4.8.0 works and 4.8.0 is a rebase so we have > > [loberman@localhost linux-stable-4.8.10]$ git log --oneline v4.7.9..v4.8 | wc -l > 14552 > > No obvious single commits stand out for me for copy_from* or copy_to* > There is this: > > 3fa6c50 mm: optimize copy_page_to/from_iter_iovec > 6e05050 sh: fix copy_from_user() > e697100 x86/uaccess: force copy_*_user() to be inlined > > I will have to do this the hard way with bisects to figure out which commit addresses this. > > Back when I have had enough time to do it. > > Thanks > Laurence Yes, in fact... Bisecting between 4.7 and 4.8 to see where the behavior changes so that the zero byte no longer appears leads to this commit: --- Bisecting: 0 revisions left to test after this (roughly 0 steps) [3fa6c507319c897598512da91c010a4ad2ed682c] mm: optimize copy_page_to/from_iter_iovec --- commit 3fa6c507319c897598512da91c010a4ad2ed682c Author: Mikulas Patocka <mpatocka@xxxxxxxxxx> Date: Thu Jul 28 15:48:50 2016 -0700 mm: optimize copy_page_to/from_iter_iovec copy_page_to_iter_iovec() and copy_page_from_iter_iovec() copy some data to userspace or from userspace. These functions have a fast path where they map a page using kmap_atomic and a slow path where they use kmap. kmap is slower than kmap_atomic, so the fast path is preferred. However, on kernels without highmem support, kmap just calls page_address, so there is no need to avoid kmap. On kernels without highmem support, the fast path just increases code size (and cache footprint) and it doesn't improve copy performance in any way. This patch enables the fast path only if CONFIG_HIGHMEM is defined. Code size reduced by this patch: x86 (without highmem) 928 x86-64 960 sparc64 848 alpha 1136 pa-risc 1200 [akpm@xxxxxxxxxxxxxxxxxxxx: use IS_ENABLED(), per Andi] Link: http://lkml.kernel.org/r/alpine.LRH.2.02.1607221711410.4818@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Cc: Johannes Weiner <hannes@xxxxxxxxxxx> Cc: Andi Kleen <andi@xxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> --- In other words, this commit made the bad behavior go away in 4.8. Need to look at this in more detail, it doesn't appear as if this patch was intended to fix such a problem. -Ewan -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
