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Fri, 09 Aug 2024 01:13:47 +0200 Message-ID: <1197adb6-332f-4b93-a614-fe78f74f1e9e@gmx.com> Date: Fri, 9 Aug 2024 08:43:44 +0930 Precedence: bulk X-Mailing-List: linux-btrfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] btrfs: introduce extent_map::em_cached member To: Filipe Manana , Qu Wenruo Cc: linux-btrfs@vger.kernel.org References: <3843c1c37c19f547236a8ec2690d9b258a1c4489.1723096922.git.wqu@suse.com> Content-Language: en-US From: Qu Wenruo Autocrypt: addr=quwenruo.btrfs@gmx.com; keydata= xsBNBFnVga8BCACyhFP3ExcTIuB73jDIBA/vSoYcTyysFQzPvez64TUSCv1SgXEByR7fju3o 8RfaWuHCnkkea5luuTZMqfgTXrun2dqNVYDNOV6RIVrc4YuG20yhC1epnV55fJCThqij0MRL 1NxPKXIlEdHvN0Kov3CtWA+R1iNN0RCeVun7rmOrrjBK573aWC5sgP7YsBOLK79H3tmUtz6b 9Imuj0ZyEsa76Xg9PX9Hn2myKj1hfWGS+5og9Va4hrwQC8ipjXik6NKR5GDV+hOZkktU81G5 gkQtGB9jOAYRs86QG/b7PtIlbd3+pppT0gaS+wvwMs8cuNG+Pu6KO1oC4jgdseFLu7NpABEB AAHNIlF1IFdlbnJ1byA8cXV3ZW5ydW8uYnRyZnNAZ214LmNvbT7CwJQEEwEIAD4CGwMFCwkI BwIGFQgJCgsCBBYCAwECHgECF4AWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCY00iVQUJDToH pgAKCRDCPZHzoSX+qNKACACkjDLzCvcFuDlgqCiS4ajHAo6twGra3uGgY2klo3S4JespWifr BLPPak74oOShqNZ8yWzB1Bkz1u93Ifx3c3H0r2vLWrImoP5eQdymVqMWmDAq+sV1Koyt8gXQ XPD2jQCrfR9nUuV1F3Z4Lgo+6I5LjuXBVEayFdz/VYK63+YLEAlSowCF72Lkz06TmaI0XMyj jgRNGM2MRgfxbprCcsgUypaDfmhY2nrhIzPUICURfp9t/65+/PLlV4nYs+DtSwPyNjkPX72+ LdyIdY+BqS8cZbPG5spCyJIlZonADojLDYQq4QnufARU51zyVjzTXMg5gAttDZwTH+8LbNI4 mm2YzsBNBFnVga8BCACqU+th4Esy/c8BnvliFAjAfpzhI1wH76FD1MJPmAhA3DnX5JDORcga CbPEwhLj1xlwTgpeT+QfDmGJ5B5BlrrQFZVE1fChEjiJvyiSAO4yQPkrPVYTI7Xj34FnscPj /IrRUUka68MlHxPtFnAHr25VIuOS41lmYKYNwPNLRz9Ik6DmeTG3WJO2BQRNvXA0pXrJH1fN GSsRb+pKEKHKtL1803x71zQxCwLh+zLP1iXHVM5j8gX9zqupigQR/Cel2XPS44zWcDW8r7B0 q1eW4Jrv0x19p4P923voqn+joIAostyNTUjCeSrUdKth9jcdlam9X2DziA/DHDFfS5eq4fEv ABEBAAHCwHwEGAEIACYCGwwWIQQt33LlpaVbqJ2qQuHCPZHzoSX+qAUCY00ibgUJDToHvwAK CRDCPZHzoSX+qK6vB/9yyZlsS+ijtsvwYDjGA2WhVhN07Xa5SBBvGCAycyGGzSMkOJcOtUUf tD+ADyrLbLuVSfRN1ke738UojphwkSFj4t9scG5A+U8GgOZtrlYOsY2+cG3R5vjoXUgXMP37 INfWh0KbJodf0G48xouesn08cbfUdlphSMXujCA8y5TcNyRuNv2q5Nizl8sKhUZzh4BascoK DChBuznBsucCTAGrwPgG4/ul6HnWE8DipMKvkV9ob1xJS2W4WJRPp6QdVrBWJ9cCdtpR6GbL iQi22uZXoSPv/0oUrGU+U5X4IvdnvT+8viPzszL5wXswJZfqfy8tmHM85yjObVdIG6AlnrrD In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable X-Provags-ID: V03:K1:/9KvSUqgc8SjLSDOV59r17slfyEpcnV3ayWdak5QhExtyr/ny0q 1V6bQ5d6Shg4UvsRHWCEskoUl4OnnaqBE/aN9mIXLSxvuKS86tMUgUoEWmFL1HMPqhZ1ZW2 9QuK4mI2OgRxrJ+AIwT8sfB9g8tRRKgU38V0vm2Fg0u2NWNnZrqNGselwIgRWt8kVJI0J4d EVTgsXi121QiUiqGp1ILA== X-Spam-Flag: NO UI-OutboundReport: notjunk:1;M01:P0:U1gNdjgFfCI=;XwQemGJpFJHY5XmsJ48/D+nAhcE b40mbeon/LAh4LL8FFXNIFo7GJ4dr/Gew5u/oVBbeheAFXERUld+MtYG8qZrVdNokcfOg/srj mBib8PbJgWj144p7oL1WGkD7ZPAwfVxWKJDPCQW/LUbEZxlnR5nzpSeJ4ygQ5xzUvsnFxcKOP 6NHmERgj4Gkd5vSFK6th8q086IPuEjJ9ax3V8gTdvTHPDBxdTdG63QdDW7NvU3eRn91/C15/b ho3dRtsSSR1dJypa3MkhZOQp+FnUp+gRhrEiXVNB2MpDqP9Lor8LOqzry8itJVHtseOMLurGa e3sQANHGx8SZvGBxprjST82C3GoLzDWUh8hj8N5jt0E7D/ZPKP4q56CWBK3vkPi2sz34490b4 hUf+6IXBB8rWL6EmLGtU+RsEq+LuZsod7IRZzpZynEPomF0wBdWkF/6kXAs+94pauI0x7R313 K9fz0uzjCmOcKxzph4xGdsmdWx985ecMj0tSC6lTqg/eRe96WGPRaI4ECfn0cbrvon9MOitaF tgyySIwZT7PjWMy1hURlrvcSHQFqgyOEjdzIDLkC/ZzELV5rb11CnjMu90Tkbg0AU1qPy1iYH 6RXkmjpSgnpqEo0SFboO/ydqUXK7RCt2UADc3yQMP8ItcOdhuHuBYXJqIwZMRv+2AEZtMe5S9 VwQK9JtqEPLSc9xuh/W8UjeBzjX0BQZzfgQmdVAEQ6aL6pVKCf5jq3VIVHHbvboD+BbhsS9+m b/1i3qCiC9QqXMxaFBd2lg8Y3fzyC8LqC5tRx2yM7tBJD2RrBDLGuDnjFRRWnVWxdc5vEBmMl qmUQAHHlyVWJxTLLlyz/SWsw== =E5=9C=A8 2024/8/8 21:06, Filipe Manana =E5=86=99=E9=81=93: > On Thu, Aug 8, 2024 at 7:06=E2=80=AFAM Qu Wenruo wrote: >> >> For data read, we always pass an extent_map pointer as @em_cached for >> btrfs_readpage(). >> And that @em_cached pointer has the same lifespan as the @bio_ctrl we >> passed in, so it's a perfect match to move @em_cached into @bio_ctrl. >> >> Signed-off-by: Qu Wenruo >> --- >> fs/btrfs/extent_io.c | 19 +++++++++---------- >> 1 file changed, 9 insertions(+), 10 deletions(-) >> >> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c >> index 822e2bf8bc99..d4ad98488c03 100644 >> --- a/fs/btrfs/extent_io.c >> +++ b/fs/btrfs/extent_io.c >> @@ -101,6 +101,8 @@ struct btrfs_bio_ctrl { >> blk_opf_t opf; >> btrfs_bio_end_io_t end_io_func; >> struct writeback_control *wbc; >> + /* For read/write extent map cache. */ >> + struct extent_map *em_cached; > > As mentioned before, this can be named just "em", it's clear enough > given the comment and the fact the structure is contextual. > The naming "em_cached" is odd, and would be more logical as > "cached_em" if it were outside the structure. > Definitely will fix the naming and subjective line. [...] > > So instead of calling free_extent_map() before each submit_one_bio() > call, it would be better to do the call inside submit_one_bio() and > then set bio_ctrl->em_cached to NULL there, > to avoid any use-after-free surprises and leaks in case someone later > forgets to call free_extent_map(). Also removes duplicated code. This is not that straightforward. E.g. |<- em 1 ->|<- em 2 ->| | 1 | 2 | 3 | 4 | <- Pages For page 1, we got em1, and now bio_ctrl->em =3D em1, add the page into the bio_ctrl->bio. For page 2, the same, we just reuse bio_ctrl->em, add the page into bio_ctrl->bio. For page 3, we find bio_ctrl->em no longer matches our range, so we get the new em2 and stored it into bio_ctrl->em, so far so good. But at submit_extent_folio(), we found page 3 is not contig for bio_ctrl->bio, thus we should submit the bio. Remember the bio_ctrl->em is now em2, if we clear it right now, the next page, page 4 will still need to do an em lookup. So unfortunately we can not free the cached em at submit_one_bio(), as at that timing, our cached em can be switched to the next range. This is not a huge deal, as we're only doing at most 2 em lookups per dirty range, but it's still not the ideal 1 em lookup of the existing code= . And considering there are only 4 call sites allocating an on-stack bio_ctrl, the explicit free_extent_map() call is not that a big deal. Thanks, Qu > > Also, as mentioned before, the subject of the patch is incorrect, it > should mention btrfs_bio_ctrl:: instead of extent_map:: > > Thanks. > >> submit_one_bio(&bio_ctrl); >> } > >> >> -- >> 2.45.2 >> >> >