From: Steven Rostedt <rostedt@goodmis.org> To: Gao Xiang <hsiangkao@aol.com> Cc: Amir Goldstein <amir73il@gmail.com>, devel@driverdev.osuosl.org, Stephen Rothwell <sfr@canb.auug.org.au>, Matthew Wilcox <willy@infradead.org>, Theodore Ts'o <tytso@mit.edu>, Will Deacon <will@kernel.org>, Peter Zijlstra <peterz@infradead.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Miao Xie <miaoxie@huawei.com>, linux-erofs@lists.ozlabs.org, LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>, Alexander Viro <viro@zeniv.linux.org.uk>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Linus Torvalds <torvalds@linux-foundation.org> Subject: Re: [PATCH v3 12/24] erofs: introduce tagged pointer Date: Mon, 22 Jul 2019 12:35:02 -0400 [thread overview] Message-ID: <20190722123502.328cecb6@gandalf.local.home> (raw) In-Reply-To: <0c2cdd4f-8fe7-6084-9c2d-c2e475e6806e@aol.com> On Mon, 22 Jul 2019 23:33:53 +0800 Gao Xiang <hsiangkao@aol.com> wrote: > Hi Steven, > > On 2019/7/22 ????10:40, Steven Rostedt wrote: > >>> and I'm not sure Al could accept __fdget conversion (I just wanted to give a example then...) > >>> > >>> Therefore, I tend to keep silence and just promote EROFS... some better ideas?... > >>> > >> Writing example conversion patches to demonstrate cleaner code > >> and perhaps reduce LOC seems the best way. > > Yes, I would be more interested in seeing patches that clean up the > > code than just talking about it. > > > > I guess that is related to me, though I didn't plan to promote > a generic tagged pointer implementation in this series... I don't expect you to either. > > I try to describe what erofs met and my own implementation, > assume that we have 3 tagged pointers, a, b, c, and one > potential user only (no need to ACCESS_ONCE). > > One way is > > #define A_MASK 1 > #define B_MASK 1 > #define C_MASK 3 > > /* now we have 3 mask there, A, B, C is simple, > the real name could be long... */ > > void *a; > void *b; > void *c; /* and some pointers */ > > In order to decode the tag, we have to > ((unsigned long)a & A_MASK) > > to decode the ptr, we have to > ((unsigned long)a & ~A_MASK) > > In order to fold the tagged pointer... > (void *)((unsigned long)a | tag) And you need a way to clear the flag. > > You can see the only meaning of these masks is the bitlength of tags, > but there are many masks (or we have to do open-coded a & 3, > if bitlength is changed, we have to fix them all)... > > therefore my approach is > > typedef tagptr1_t ta; /* tagptr type a with 1-bit tag */ > typedef tagptr1_t tb; /* tagptr type b with 1-bit tag */ > typedef tagptr2_t tc; /* tagptr type c with 2-bit tag */ > > and ta a; tb b; tc c; > > the type will represent its bitlength of tags and we can use ta, tb, tc > to avoid masks or open-coded bitlength. > > In order to decode the tag, we can > tagptr_unfold_tags(a) > > In order to decode the ptr, we can > tagptr_unfold_ptr(a) > > In order to fold the tagged pointer... > a = tagptr_fold(ta, ptr, tag) > > > ACCESS_ONCE stuff is another thing... If my approach seems cleaner, > we could move to include/linux later after EROFS stuffs is done... > Or I could use a better tagptr approach later if any... Looking at the ring buffer code, it may be a bit too complex to try to use a generic infrastructure. Look at rb_head_page_set(), where it does a cmpxchg to set or clear the flags and then tests the previous flags to know what actions need to be done. The ring buffer tag code was added in 2009, the rtmutex tag code was added in 2006. It's been 10 years before we needed another tag operation. I'm not sure we benefit from making this generic. -- Steve
WARNING: multiple messages have this Message-ID (diff)
From: rostedt@goodmis.org (Steven Rostedt) Subject: [PATCH v3 12/24] erofs: introduce tagged pointer Date: Mon, 22 Jul 2019 12:35:02 -0400 [thread overview] Message-ID: <20190722123502.328cecb6@gandalf.local.home> (raw) In-Reply-To: <0c2cdd4f-8fe7-6084-9c2d-c2e475e6806e@aol.com> On Mon, 22 Jul 2019 23:33:53 +0800 Gao Xiang <hsiangkao@aol.com> wrote: > Hi Steven, > > On 2019/7/22 ????10:40, Steven Rostedt wrote: > >>> and I'm not sure Al could accept __fdget conversion (I just wanted to give a example then...) > >>> > >>> Therefore, I tend to keep silence and just promote EROFS... some better ideas?... > >>> > >> Writing example conversion patches to demonstrate cleaner code > >> and perhaps reduce LOC seems the best way. > > Yes, I would be more interested in seeing patches that clean up the > > code than just talking about it. > > > > I guess that is related to me, though I didn't plan to promote > a generic tagged pointer implementation in this series... I don't expect you to either. > > I try to describe what erofs met and my own implementation, > assume that we have 3 tagged pointers, a, b, c, and one > potential user only (no need to ACCESS_ONCE). > > One way is > > #define A_MASK 1 > #define B_MASK 1 > #define C_MASK 3 > > /* now we have 3 mask there, A, B, C is simple, > the real name could be long... */ > > void *a; > void *b; > void *c; /* and some pointers */ > > In order to decode the tag, we have to > ((unsigned long)a & A_MASK) > > to decode the ptr, we have to > ((unsigned long)a & ~A_MASK) > > In order to fold the tagged pointer... > (void *)((unsigned long)a | tag) And you need a way to clear the flag. > > You can see the only meaning of these masks is the bitlength of tags, > but there are many masks (or we have to do open-coded a & 3, > if bitlength is changed, we have to fix them all)... > > therefore my approach is > > typedef tagptr1_t ta; /* tagptr type a with 1-bit tag */ > typedef tagptr1_t tb; /* tagptr type b with 1-bit tag */ > typedef tagptr2_t tc; /* tagptr type c with 2-bit tag */ > > and ta a; tb b; tc c; > > the type will represent its bitlength of tags and we can use ta, tb, tc > to avoid masks or open-coded bitlength. > > In order to decode the tag, we can > tagptr_unfold_tags(a) > > In order to decode the ptr, we can > tagptr_unfold_ptr(a) > > In order to fold the tagged pointer... > a = tagptr_fold(ta, ptr, tag) > > > ACCESS_ONCE stuff is another thing... If my approach seems cleaner, > we could move to include/linux later after EROFS stuffs is done... > Or I could use a better tagptr approach later if any... Looking at the ring buffer code, it may be a bit too complex to try to use a generic infrastructure. Look at rb_head_page_set(), where it does a cmpxchg to set or clear the flags and then tests the previous flags to know what actions need to be done. The ring buffer tag code was added in 2009, the rtmutex tag code was added in 2006. It's been 10 years before we needed another tag operation. I'm not sure we benefit from making this generic. -- Steve
next prev parent reply other threads:[~2019-07-22 16:35 UTC|newest] Thread overview: 92+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-07-22 2:50 [PATCH v3 00/24] erofs: promote erofs from staging Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 01/24] erofs: add on-disk layout Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 3:26 ` Stephen Rothwell 2019-07-22 3:26 ` Stephen Rothwell 2019-07-22 3:26 ` Stephen Rothwell 2019-07-22 3:29 ` Gao Xiang 2019-07-22 3:29 ` Gao Xiang 2019-07-22 5:05 ` Greg Kroah-Hartman 2019-07-22 5:05 ` Greg Kroah-Hartman 2019-07-22 5:24 ` Gao Xiang 2019-07-22 5:24 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 02/24] erofs: add erofs in-memory stuffs Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 03/24] erofs: add super block operations Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 04/24] erofs: add raw address_space operations Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 05/24] erofs: add inode operations Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 06/24] erofs: support special inode Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 07/24] erofs: add directory operations Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 08/24] erofs: add namei functions Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 09/24] erofs: support tracepoint Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 10/24] erofs: update Kconfig and Makefile Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 11/24] erofs: introduce xattr & posixacl support Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 12/24] erofs: introduce tagged pointer Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 4:39 ` Amir Goldstein 2019-07-22 4:39 ` Amir Goldstein 2019-07-22 5:01 ` Gao Xiang 2019-07-22 5:01 ` Gao Xiang 2019-07-22 6:16 ` Amir Goldstein 2019-07-22 6:16 ` Amir Goldstein 2019-07-22 6:31 ` Gao Xiang 2019-07-22 6:31 ` Gao Xiang 2019-07-22 14:40 ` Steven Rostedt 2019-07-22 14:40 ` Steven Rostedt 2019-07-22 15:33 ` Gao Xiang 2019-07-22 15:33 ` Gao Xiang 2019-07-22 16:35 ` Steven Rostedt [this message] 2019-07-22 16:35 ` Steven Rostedt 2019-07-22 16:52 ` Gao Xiang 2019-07-22 16:52 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 13/24] erofs: add compression indexes support Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 14/24] erofs: introduce superblock registration Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 15/24] erofs: introduce erofs shrinker Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 16/24] erofs: introduce workstation for decompression Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 17/24] erofs: introduce per-CPU buffers implementation Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 18/24] erofs: introduce pagevec for decompression subsystem Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 19/24] erofs: add erofs_allocpage() Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 20/24] erofs: introduce generic decompression backend Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 21/24] erofs: introduce LZ4 decompression inplace Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 22/24] erofs: introduce the decompression frontend Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 23/24] erofs: introduce cached decompression Gao Xiang 2019-07-22 2:50 ` Gao Xiang 2019-07-22 10:18 ` David Sterba 2019-07-22 10:18 ` David Sterba 2019-07-22 10:58 ` Gao Xiang 2019-07-22 10:58 ` Gao Xiang 2019-07-22 13:25 ` Theodore Y. Ts'o 2019-07-22 13:25 ` Theodore Y. Ts'o 2019-07-22 14:16 ` Gao Xiang 2019-07-22 14:16 ` Gao Xiang 2019-07-22 15:12 ` Theodore Y. Ts'o 2019-07-22 15:12 ` Theodore Y. Ts'o 2019-07-22 16:27 ` Gao Xiang 2019-07-22 16:27 ` Gao Xiang 2019-07-23 12:31 ` David Sterba 2019-07-23 12:31 ` David Sterba 2019-07-23 13:30 ` Gao Xiang 2019-07-23 13:30 ` Gao Xiang 2019-07-22 2:50 ` [PATCH v3 24/24] erofs: add document Gao Xiang 2019-07-22 2:50 ` Gao Xiang
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