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[184.145.4.219]) by smtp.gmail.com with ESMTPSA id y1sm498232qkp.21.2021.05.26.18.44.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 May 2021 18:44:23 -0700 (PDT) Date: Wed, 26 May 2021 21:44:22 -0400 From: Peter Xu To: Alistair Popple Cc: linux-mm@kvack.org, akpm@linux-foundation.org, nouveau@lists.freedesktop.org, bskeggs@redhat.com, rcampbell@nvidia.com, linux-doc@vger.kernel.org, jhubbard@nvidia.com, bsingharora@gmail.com, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, hch@infradead.org, jglisse@redhat.com, willy@infradead.org, jgg@nvidia.com, hughd@google.com Subject: Re: [PATCH v9 06/10] mm/memory.c: Allow different return codes for copy_nonpresent_pte() Message-ID: References: <20210524132725.12697-1-apopple@nvidia.com> <20210524132725.12697-7-apopple@nvidia.com> <2005328.bFqPmhE5MS@nvdebian> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <2005328.bFqPmhE5MS@nvdebian> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Thu, May 27, 2021 at 11:20:36AM +1000, Alistair Popple wrote: > On Thursday, 27 May 2021 5:50:05 AM AEST Peter Xu wrote: > > On Mon, May 24, 2021 at 11:27:21PM +1000, Alistair Popple wrote: > > > Currently if copy_nonpresent_pte() returns a non-zero value it is > > > assumed to be a swap entry which requires further processing outside the > > > loop in copy_pte_range() after dropping locks. This prevents other > > > values being returned to signal conditions such as failure which a > > > subsequent change requires. > > > > > > Instead make copy_nonpresent_pte() return an error code if further > > > processing is required and read the value for the swap entry in the main > > > loop under the ptl. > > > > > > Signed-off-by: Alistair Popple > > > > > > --- > > > > > > v9: > > > > > > New for v9 to allow device exclusive handling to occur in > > > copy_nonpresent_pte(). > > > --- > > > > > > mm/memory.c | 12 +++++++----- > > > 1 file changed, 7 insertions(+), 5 deletions(-) > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > index 2fb455c365c2..e061cfa18c11 100644 > > > --- a/mm/memory.c > > > +++ b/mm/memory.c > > > @@ -718,7 +718,7 @@ copy_nonpresent_pte(struct mm_struct *dst_mm, struct > > > mm_struct *src_mm,> > > > if (likely(!non_swap_entry(entry))) { > > > > > > if (swap_duplicate(entry) < 0) > > > > > > - return entry.val; > > > + return -EAGAIN; > > > > > > /* make sure dst_mm is on swapoff's mmlist. */ > > > if (unlikely(list_empty(&dst_mm->mmlist))) { > > > > > > @@ -974,11 +974,13 @@ copy_pte_range(struct vm_area_struct *dst_vma, > > > struct vm_area_struct *src_vma,> > > > continue; > > > > > > } > > > if (unlikely(!pte_present(*src_pte))) { > > > > > > - entry.val = copy_nonpresent_pte(dst_mm, src_mm, > > > - dst_pte, src_pte, > > > - src_vma, addr, rss); > > > - if (entry.val) > > > + ret = copy_nonpresent_pte(dst_mm, src_mm, > > > + dst_pte, src_pte, > > > + src_vma, addr, rss); > > > + if (ret == -EAGAIN) { > > > + entry = pte_to_swp_entry(*src_pte); > > > > > > break; > > > > > > + } > > > > > > progress += 8; > > > continue; > > > > > > } > > > > Note that -EAGAIN was previously used by copy_present_page() for early cow > > use. Here later although we check entry.val first: > > > > if (entry.val) { > > if (add_swap_count_continuation(entry, GFP_KERNEL) < 0) { > > ret = -ENOMEM; > > goto out; > > } > > entry.val = 0; > > } else if (ret) { > > WARN_ON_ONCE(ret != -EAGAIN); > > prealloc = page_copy_prealloc(src_mm, src_vma, addr); > > if (!prealloc) > > return -ENOMEM; > > /* We've captured and resolved the error. Reset, try again. > > */ ret = 0; > > } > > > > We didn't reset "ret" in entry.val case (maybe we should?). Then in the next > > round of "goto again" if "ret" is unluckily untouched, it could reach the > > 2nd if check, and I think it could cause an unexpected > > page_copy_prealloc(). > > Thanks, I had considered that but saw "ret" was always set either by > copy_nonpresent_pte() or copy_present_pte(). However missed the "unlucky" case > at the start of the loop: > > if (progress >= 32) { > progress = 0; > if (need_resched() || > spin_needbreak(src_ptl) || pin_needbreak(dst_ptl)) > break; > > Looking at this again though checking different variables to figure out what > to do outside the locks and reusing error codes seems error prone. I reused - > EAGAIN for copy_nonpresent_pte() simply because that seemed the most sensible > error code, but I don't think that aids readability and it might be better to > use a unique error code for each case needing extra handling. > > So it might be better if I update this patch to: > 1) Use unique error codes for each case requiring special handling outside the > lock. > 2) Only check "ret" to determine what to do outside locks (ie. not entry.val) > 3) Document these. > 4) Always reset ret after handling. > > Thoughts? Looks good to me. Thanks, -- Peter Xu