linux-kernel - Re: [PATCH] mm/filemap: add static for function __add_to_page_cache

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Date:   Wed, 9 Dec 2020 15:46:28 +0100
From:   Stanislaw Gruszka <stf_xl@...pl>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Michal Kubecek <mkubecek@...e.cz>,
        Justin Forbes <jmforbes@...uxtx.org>,
        bpf <bpf@...r.kernel.org>, Alex Shi <alex.shi@...ux.alibaba.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Josef Bacik <josef@...icpanda.com>
Subject: Re: [PATCH] mm/filemap: add static for function
 __add_to_page_cache_locked

On Mon, Dec 07, 2020 at 05:12:52PM -0800, Alexei Starovoitov wrote:
> > > > > > > >>> -noinline int __add_to_page_cache_locked(struct page *page,
> > > > > > > >>> +static noinline int __add_to_page_cache_locked(struct page *page,
> > > > > > > >>>                                         struct address_space *mapping,
> > > > > > > >>>                                         pgoff_t offset, gfp_t gfp,
> > > > > > > >>>                                         void **shadowp)
> > > > > > > >
> > > > > > > > It's unclear to me whether BTF_ID() requires that the target symbol be
> > > > > > > > non-static.  It doesn't actually reference the symbol:
> > > > > > > >
> > > > > > > > #define BTF_ID(prefix, name) \
> > > > > > > >         __BTF_ID(__ID(__BTF_ID__##prefix##__##name##__))

[snip]

> > > __add_to_page_cache_locked") made the function static which breaks the
> > > build in btfids phase - but it seems to happen only on some
> > > architectures. In our case, ppc64, ppc64le and riscv64 are broken,
> > > x86_64, i586 and s390x succeed. (I made a mistake above, aarch64 did not
> > > fail - but only because it was not built at all.)
> > >
> > > The thread starts with
> > > http://lkml.kernel.org/r/1604661895-5495-1-git-send-email-alex.shi@linux.alibaba.com

I have 5.10-rc7 build failure because of this on x86_64:

  BTFIDS  vmlinux
FAILED unresolved symbol __add_to_page_cache_locked
make: *** [Makefile:1170: vmlinux] Error 255

> > Got it. So the above commit is wrong.
> > The addition of "static" is incorrect here.
> > Regardless of btf_id generation.
> > "static noinline" means that the error injection in that spot is unreliable.
> > Even when bpf is completely compiled out of the kernel.
> 
> I finally realized that the addition of 'static' was pushed into Linus's tree :(
> Please revert commit 3351b16af494 ("mm/filemap: add static for
> function __add_to_page_cache_locked")

At this point of release cycle we should probably go with revert,
but I think the main problem is that BPF and ERROR_INJECTION use
function that is not intended to be used externally. For external users
add_to_page_cache_lru() and add_to_page_cache_locked() are exported
and I think those should be used (see the patch below).

Stanislaw

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1388bf733071..dd6357802504 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11487,7 +11487,8 @@ BTF_SET_START(btf_non_sleepable_error_inject)
 /* Three functions below can be called from sleepable and non-sleepable context.
  * Assume non-sleepable from bpf safety point of view.
  */
-BTF_ID(func, __add_to_page_cache_locked)
+BTF_ID(func, add_to_page_cache_locked)
+BTF_ID(func, add_to_page_cache_lru)
 BTF_ID(func, should_fail_alloc_page)
 BTF_ID(func, should_failslab)
 BTF_SET_END(btf_non_sleepable_error_inject)
diff --git a/mm/filemap.c b/mm/filemap.c
index 331f4261d723..168deec64a10 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -827,10 +827,10 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
 }
 EXPORT_SYMBOL_GPL(replace_page_cache_page);
 
-static noinline int __add_to_page_cache_locked(struct page *page,
-					struct address_space *mapping,
-					pgoff_t offset, gfp_t gfp,
-					void **shadowp)
+static int __add_to_page_cache_locked(struct page *page,
+				      struct address_space *mapping,
+				      pgoff_t offset, gfp_t gfp,
+				      void **shadowp)
 {
 	XA_STATE(xas, &mapping->i_pages, offset);
 	int huge = PageHuge(page);
@@ -907,7 +907,6 @@ static noinline int __add_to_page_cache_locked(struct page *page,
 	put_page(page);
 	return error;
 }
-ALLOW_ERROR_INJECTION(__add_to_page_cache_locked, ERRNO);
 
 /**
  * add_to_page_cache_locked - add a locked page to the pagecache
@@ -928,6 +927,7 @@ int add_to_page_cache_locked(struct page *page, struct address_space *mapping,
 					  gfp_mask, NULL);
 }
 EXPORT_SYMBOL(add_to_page_cache_locked);
+ALLOW_ERROR_INJECTION(add_to_page_cache_locked, ERRNO);
 
 int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
 				pgoff_t offset, gfp_t gfp_mask)
@@ -957,6 +957,7 @@ int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
 	return ret;
 }
 EXPORT_SYMBOL_GPL(add_to_page_cache_lru);
+ALLOW_ERROR_INJECTION(add_to_page_cache_lru, ERRNO);
 
 #ifdef CONFIG_NUMA
 struct page *__page_cache_alloc(gfp_t gfp)