man/man2/cachestat.2: Add page

The text was converted from the commit message in linux.git
cf264e1329fb (2023-06-09; "cachestat: implement cachestat syscall").

Link: <https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cf264e1329fb0307e044f7675849f9f38b44c11a>
Signed-off-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Matteo Croce <teknoraver@meta.com>
Message-ID: <20250611141350.1829-1-technoboy85@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

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+.\" Copyright, the authors of the Linux man-pages project
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH cachestat 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+cachestat \- query the page cache statistics of a file
+.SH SYNOPSIS
+.nf
+.B #include <sys/mman.h>
+.P
+.BI "int cachestat(unsigned int " fd ", struct cachestat_range *" cstat_range ,
+.BI "              struct cachestat *" cstat ", unsigned int " flags );
+.fi
+.P
+.EX
+.B struct cachestat_range {
+.B "    __u64 off;"
+.B "    __u64 len;"
+.B };
+.P
+.B struct cachestat {
+.B "    __u64 nr_cache;"
+.B "    __u64 nr_dirty;"
+.B "    __u64 nr_writeback;"
+.B "    __u64 nr_evicted;"
+.B "    __u64 nr_recently_evicted;"
+.B };
+.EE
+.SH DESCRIPTION
+.B cachestat()
+queries the number of cached pages, dirty pages,
+pages marked for writeback, evicted pages,
+and recently evicted pages in the byte range specified by
+.I .off
+and
+.I .len
+in the
+.B cachestat_range
+structure.
+.P
+An evicted page is one that was previously in the page cache
+but has since been evicted.
+A page is considered recently evicted if its reentry into the cache
+would indicate active usage under memory pressure.
+.P
+The results are returned in a
+.B cachestat
+structure, pointed to by the
+.I cstat
+argument.
+.P
+The
+.I .off
+and
+.I .len
+fields must be non-negative.
+If
+.IR .len\~>\~0 ,
+the queried range is
+.RI [ .off ,\~ .off+.len ].
+If
+.IR len\~==\~0 ,
+the range is from
+.I .off
+to the end of the file.
+.P
+The
+.I flags
+argument is reserved for future use and must be set to
+.BR 0 .
+.P
+Currently,
+.B hugetlbfs
+files are not supported.
+.SH RETURN VALUE
+On success,
+.B cachestat()
+returns 0.
+On error, \-1 is returned,
+and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EFAULT
+.I cstat
+or
+.I cstat_range
+point to an invalid address.
+.TP
+.B EINVAL
+Invalid
+.I flags
+value.
+.TP
+.B EBADF
+Invalid file descriptor.
+.TP
+.B EOPNOTSUPP
+The file descriptor refers to a
+.B hugetlbfs
+file, which is unsupported.
+.SH STANDARDS
+Linux.
+.SH HISTORY
+Linux 6.5.
+.SH CAVEATS
+Note that the status of a page may change after
+.B cachestat()
+retrieves it but before the values are returned to the application;
+thus, the values may be slightly outdated.