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From: "Lance Yang" Message-ID: <1c81b176893c349dfb12feaebd17e0694fedec0b@linux.dev> TLS-Required: No Subject: Re: [RESEND PATCH v2 1/1] mm/rmap: add CONFIG_MM_ID guard for folio_test_large_maybe_mapped_shared() To: "Andrew Morton" , "David Hildenbrand" Cc: mingzhe.yang@ly.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Lance Yang" In-Reply-To: <641755b75b4ecb9c9822e15e707a0ebf1e250788@linux.dev> References: <20250418152228.20545-1-lance.yang@linux.dev> <20250420162925.2c58c018defee9ee192be553@linux-foundation.org> <641755b75b4ecb9c9822e15e707a0ebf1e250788@linux.dev> X-Migadu-Flow: FLOW_OUT April 21, 2025 at 1:13 PM, "Lance Yang" wrote: >=20 >=20April 21, 2025 at 7:29 AM, "Andrew Morton" wrote: >=20 >=20>=20 >=20> On Fri, 18 Apr 2025 23:22:28 +0800 Lance Yang = wrote: > >=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20> From: Lance Yang > >=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20> To prevent folio_test_large_maybe_mapped_shared() from being used = without > >=20 >=20>=20=20 >=20>=20 >=20> CONFIG_MM_ID, we add a compile-time check rather than wrapping it = in > >=20 >=20>=20=20 >=20>=20 >=20> '#ifdef', avoiding even more #ifdef in callers that already use > >=20 >=20>=20=20 >=20>=20 >=20> IS_ENABLED(CONFIG_MM_ID). > >=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20> Also, we used plenty of IS_ENABLED() on purpose to keep the code f= ree of > >=20 >=20>=20=20 >=20>=20 >=20> '#ifdef' mess. > >=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20> I dunno, this just seems really whacky. > >=20 >=20 > I'd hope David could leave some comments on that. >=20 >=20>=20 >=20> --- a/include/linux/page-flags.h > >=20 >=20>=20=20 >=20>=20 >=20> +++ b/include/linux/page-flags.h > >=20 >=20>=20=20 >=20>=20 >=20> @@ -1232,6 +1232,8 @@ static inline int folio_has_private(const st= ruct folio *folio) > >=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20> static inline bool folio_test_large_maybe_mapped_shared(const stru= ct folio *folio) > >=20 >=20>=20=20 >=20>=20 >=20> { > >=20 >=20>=20=20 >=20>=20 >=20> + /* This function should never be called without CONFIG_MM_ID ena= bled. */ > >=20 >=20>=20=20 >=20>=20 >=20>=20=20 >=20>=20 >=20> A correcter comment would be "This function should never be compil= ed > >=20 >=20>=20=20 >=20>=20 >=20> without CONFIG_MM_ID enabled". > >=20 >=20 > Yes, that is more exact ;) >=20 >=20>=20 >=20> Which lets the cat out of the bag. Why the heck is it being compile= d > >=20 >=20>=20=20 >=20>=20 >=20> with CONFIG_MM_ID=3Dn?? We have tools to prevent that. > >=20 >=20>=20=20 >=20>=20 >=20> Can we just slap "#ifdef CONFIG_MM_ID" around the whole function? = It > >=20 >=20>=20=20 >=20>=20 >=20> should have no callers, right? If the linker ends up complaining t= hen > >=20 >=20>=20=20 >=20>=20 >=20> something went wrong. > >=20 >=20 > The reason we can't simply add #ifdef CONFIG_MM_ID around folio_test_la= rge_maybe_mapped_shared() >=20 >=20is because its caller folio_maybe_mapped_shared() relies on IS_ENABLE= D(CONFIG_MM_ID). static inline bool folio_maybe_mapped_shared(struct folio *folio) { [...] if (!IS_ENABLED(CONFIG_MM_ID)) return true; [...] return folio_test_large_maybe_mapped_shared(folio); } folio_maybe_mapped_shared() is always available - doesn't depend on CONFIG_MM_ID or CONFIG_TRANSPARENT_HUGEPAGE. Thanks, Lance >=20 >=20If we do, with CONFIG_TRANSPARENT_HUGEPAGE=3DN, we'll hit compilation= errors like: >=20 >=20./include/linux/mm.h: In function =E2=80=98folio_maybe_mapped_shared= =E2=80=99: >=20 >=20./include/linux/mm.h:2337:16: error: implicit declaration of function= =E2=80=98folio_test_large_maybe_mapped_shared=E2=80=99; did you mean =E2= =80=98folio_maybe_mapped_shared=E2=80=99? [-Werror=3Dimplicit-function-de= claration] >=20 >=20 2337 | return folio_test_large_maybe_mapped_shared(folio); >=20 >=20 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ >=20 >=20 | folio_maybe_mapped_shared >=20 >=20cc1: some warnings being treated as errors >=20 >=20Thanks, >=20 >=20Lance > >=20 >=20>=20 >=20> >