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Date:   Tue, 16 May 2023 15:02:18 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Anders Roxell <anders.roxell@linaro.org>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
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Subject: Re: [PATCH v5 3/6] mm/gup: remove vmas parameter from
 get_user_pages_remote()
Message-Id: <20230516150218.477c5e9d0a2d9ef8b057069c@linux-foundation.org>
In-Reply-To: <20230516094919.GA411@mutt>
References: <cover.1684097001.git.lstoakes@gmail.com>
        <afe323639b7bda066ee5c7a6cca906f5ad8df940.1684097002.git.lstoakes@gmail.com>
        <20230516094919.GA411@mutt>
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On Tue, 16 May 2023 11:49:19 +0200 Anders Roxell <anders.roxell@linaro.org> wrote:

> On 2023-05-14 22:26, Lorenzo Stoakes wrote:
> > The only instances of get_user_pages_remote() invocations which used the
> > vmas parameter were for a single page which can instead simply look up the
> > VMA directly. In particular:-
> > 
> > - __update_ref_ctr() looked up the VMA but did nothing with it so we simply
> >   remove it.
> > 
> > - __access_remote_vm() was already using vma_lookup() when the original
> >   lookup failed so by doing the lookup directly this also de-duplicates the
> >   code.
> > 
> > We are able to perform these VMA operations as we already hold the
> > mmap_lock in order to be able to call get_user_pages_remote().
> > 
> > As part of this work we add get_user_page_vma_remote() which abstracts the
> > VMA lookup, error handling and decrementing the page reference count should
> > the VMA lookup fail.
> > 
> > This forms part of a broader set of patches intended to eliminate the vmas
> > parameter altogether.
> > 
> > -		int bytes, ret, offset;
> > +		int bytes, offset;
> >  		void *maddr;
> > -		struct page *page = NULL;
> > +		struct vm_area_struct *vma;
> > +		struct page *page = get_user_page_vma_remote(mm, addr,
> > +							     gup_flags, &vma);
> > +
> > +		if (IS_ERR_OR_NULL(page)) {
> > +			int ret = 0;
> 
> I see the warning below when building without CONFIG_HAVE_IOREMAP_PROT set.
> 
> make --silent --keep-going --jobs=32 \
> O=/home/anders/.cache/tuxmake/builds/1244/build ARCH=arm \
> CROSS_COMPILE=arm-linux-gnueabihf- /home/anders/src/kernel/next/mm/memory.c: In function '__access_remote_vm':
> /home/anders/src/kernel/next/mm/memory.c:5608:29: warning: unused variable 'ret' [-Wunused-variable]
>  5608 |                         int ret = 0;
>       |                             ^~~

Thanks, I did the obvious.

Also s/ret/res/, as `ret' is kinda reserved for "this is what this
function will return".

--- a/mm/memory.c~mm-gup-remove-vmas-parameter-from-get_user_pages_remote-fix
+++ a/mm/memory.c
@@ -5605,11 +5605,11 @@ int __access_remote_vm(struct mm_struct
 							     gup_flags, &vma);
 
 		if (IS_ERR_OR_NULL(page)) {
-			int ret = 0;
-
 #ifndef CONFIG_HAVE_IOREMAP_PROT
 			break;
 #else
+			int res = 0;
+
 			/*
 			 * Check if this is a VM_IO | VM_PFNMAP VMA, which
 			 * we can access using slightly different code.
@@ -5617,11 +5617,11 @@ int __access_remote_vm(struct mm_struct
 			if (!vma)
 				break;
 			if (vma->vm_ops && vma->vm_ops->access)
-				ret = vma->vm_ops->access(vma, addr, buf,
+				res = vma->vm_ops->access(vma, addr, buf,
 							  len, write);
-			if (ret <= 0)
+			if (res <= 0)
 				break;
-			bytes = ret;
+			bytes = res;
 #endif
 		} else {
 			bytes = len;
_