mm/smaps: add Pss_Dirty

Pss is the sum of the sizes of clean and dirty private pages, and the
proportional sizes of clean and dirty shared pages:

 Private = Private_Dirty + Private_Clean
 Shared_Proportional = Shared_Dirty_Proportional + Shared_Clean_Proportional
 Pss = Private + Shared_Proportional

The Shared*Proportional fields are not present in smaps, so it is not
always possible to determine how much of the Pss is from dirty pages and
how much is from clean pages.  This information can be useful for
measuring memory usage for the purpose of optimisation, since clean pages
can usually be discarded by the kernel immediately while dirty pages
cannot.

The smaps routines in the kernel already have access to this data, so add
a Pss_Dirty to show it to userspace.  Pss_Clean is not added since it can
be calculated from Pss and Pss_Dirty.

Link: https://lkml.kernel.org/r/20220620081251.2928103-1-vincent.whitchurch@axis.comSigned-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/ABI/testing/procfs-smaps_rollup

@@ -22,6 +22,7 @@ Description:
 			MMUPageSize:           4 kB
 			Rss:		     884 kB
 			Pss:		     385 kB
+			Pss_Dirty:	      68 kB
 			Pss_Anon:	     301 kB
 			Pss_File:	      80 kB
 			Pss_Shmem:	       4 kB

Documentation/filesystems/proc.rst

@@ -448,6 +448,7 @@ Memory Area, or VMA) there is a series of lines such as the following::
     MMUPageSize:           4 kB
     Rss:                 892 kB
     Pss:                 374 kB
+    Pss_Dirty:             0 kB
     Shared_Clean:        892 kB
     Shared_Dirty:          0 kB
     Private_Clean:         0 kB
@@ -479,7 +480,9 @@ dirty shared and private pages in the mapping.
 The "proportional set size" (PSS) of a process is the count of pages it has
 in memory, where each page is divided by the number of processes sharing it.
 So if a process has 1000 pages all to itself, and 1000 shared with one other
-process, its PSS will be 1500.
+process, its PSS will be 1500.  "Pss_Dirty" is the portion of PSS which
+consists of dirty pages.  ("Pss_Clean" is not included, but it can be
+calculated by subtracting "Pss_Dirty" from "Pss".)
 
 Note that even a page which is part of a MAP_SHARED mapping, but has only
 a single pte mapped, i.e.  is currently used by only one process, is accounted

fs/proc/task_mmu.c

@@ -406,6 +406,7 @@ struct mem_size_stats {
 	u64 pss_anon;
 	u64 pss_file;
 	u64 pss_shmem;
+	u64 pss_dirty;
 	u64 pss_locked;
 	u64 swap_pss;
 };
@@ -427,6 +428,7 @@ static void smaps_page_accumulate(struct mem_size_stats *mss,
 		mss->pss_locked += pss;
 
 	if (dirty || PageDirty(page)) {
+		mss->pss_dirty += pss;
 		if (private)
 			mss->private_dirty += size;
 		else
@@ -808,6 +810,7 @@ static void __show_smap(struct seq_file *m, const struct mem_size_stats *mss,
 {
 	SEQ_PUT_DEC("Rss:            ", mss->resident);
 	SEQ_PUT_DEC(" kB\nPss:            ", mss->pss >> PSS_SHIFT);
+	SEQ_PUT_DEC(" kB\nPss_Dirty:      ", mss->pss_dirty >> PSS_SHIFT);
 	if (rollup_mode) {
 		/*
 		 * These are meaningful only for smaps_rollup, otherwise two of