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[184.145.4.219]) by smtp.gmail.com with ESMTPSA id g18sm2200678qke.37.2021.05.26.12.50.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 26 May 2021 12:50:06 -0700 (PDT) Date: Wed, 26 May 2021 15:50:05 -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> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210524132725.12697-7-apopple@nvidia.com> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org 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(). -- Peter Xu