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Date:   Tue, 6 Sep 2022 13:23:13 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     rjw@rjwysocki.net, oleg@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de,
        ebiederm@xmission.com, bigeasy@linutronix.de,
        Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org,
        tj@kernel.org, linux-pm@vger.kernel.org
Subject: Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic
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 <YxR5Yauhd90WN/AY@gmail.com>
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On Sun, Sep 04, 2022 at 12:09:37PM +0200, Ingo Molnar wrote:

> BTW., we should probably mark/document all PF_ holes with a PF__RESERVED 
> kind of scheme? Something simple, like:
> 
>    #define PF_NPROC_EXCEEDED	0x00001000	/* set_user() noticed that RLIMIT_NPROC was exceeded */
>    #define PF_USED_MATH		0x00002000	/* If unset the fpu must be initialized before use */
>  + #define PF__RESERVED_04000	0x00004000	/* Unused */
>    #define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */
>  + #define PF__RESERVED_10000	0x00010000	/* Unused */
>    #define PF_KSWAPD		0x00020000	/* I am kswapd */
>    #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
>    #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */


How's this then, it immediately shows how holey it is :-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1722,7 +1722,9 @@ extern struct pid *cad_pid;
 #define PF_MEMALLOC		0x00000800	/* Allocating memory */
 #define PF_NPROC_EXCEEDED	0x00001000	/* set_user() noticed that RLIMIT_NPROC was exceeded */
 #define PF_USED_MATH		0x00002000	/* If unset the fpu must be initialized before use */
+#define PF__HOLE__00004000	0x00004000	/* A HOLE */
 #define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */
+#define PF__HOLE__00010000	0x00010000	/* A HOLE */
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
@@ -1730,9 +1732,14 @@ extern struct pid *cad_pid;
 						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
+#define PF__HOLE__00800000	0x00800000	/* A HOLE */
+#define PF__HOLE__01000000	0x01000000	/* A HOLE */
+#define PF__HOLE__02000000	0x02000000	/* A HOLE */
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
 #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
 #define PF_MEMALLOC_PIN		0x10000000	/* Allocation context constrained to zones which allow long term pinning. */
+#define PF__HOLE__20000000	0x20000000	/* A HOLE */
+#define PF__HOLE__40000000	0x40000000	/* A HOLE */
 #define PF_SUSPEND_TASK		0x80000000      /* This thread called freeze_processes() and should not be frozen */
 
 /*