ARC: Cache arc_c value during arc_evict()

Since arc_evict() run can take some time, arc_c change during it
may result in undesired shift in ARC states balance. Primarily in
case of arc_c reduction it may cause eviction from MFU data state
despite its being below the target already.  Instead we should
evict as originally planned and if needed do another round after.

Reviewed-by: Theera K. <tkittich@hotmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#16576
Closes openzfs#16605

module/zfs/arc.c

@@ -4240,7 +4240,7 @@ arc_evict_adj(uint64_t frac, uint64_t total, uint64_t up, uint64_t down,
 static uint64_t
 arc_evict(void)
 {
-	uint64_t asize, bytes, total_evicted = 0;
+	uint64_t bytes, total_evicted = 0;
 	int64_t e, mrud, mrum, mfud, mfum, w;
 	static uint64_t ogrd, ogrm, ogfd, ogfm;
 	static uint64_t gsrd, gsrm, gsfd, gsfm;
@@ -4277,8 +4277,9 @@ arc_evict(void)
 	arc_pd = arc_evict_adj(arc_pd, gsrd + gsfd, grd, gfd, 100);
 	arc_pm = arc_evict_adj(arc_pm, gsrm + gsfm, grm, gfm, 100);
 
-	asize = aggsum_value(&arc_sums.arcstat_size);
-	int64_t wt = t - (asize - arc_c);
+	uint64_t asize = aggsum_value(&arc_sums.arcstat_size);
+	uint64_t ac = arc_c;
+	int64_t wt = t - (asize - ac);
 
 	/*
 	 * Try to reduce pinned dnodes if more than 3/4 of wanted metadata
@@ -4303,15 +4304,15 @@ arc_evict(void)
 
 	/* Evict MRU metadata. */
 	w = wt * (int64_t)(arc_meta * arc_pm >> 48) >> 16;
-	e = MIN((int64_t)(asize - arc_c), (int64_t)(mrum - w));
+	e = MIN((int64_t)(asize - ac), (int64_t)(mrum - w));
 	bytes = arc_evict_impl(arc_mru, ARC_BUFC_METADATA, e);
 	total_evicted += bytes;
 	mrum -= bytes;
 	asize -= bytes;
 
 	/* Evict MFU metadata. */
 	w = wt * (int64_t)(arc_meta >> 16) >> 16;
-	e = MIN((int64_t)(asize - arc_c), (int64_t)(m - bytes - w));
+	e = MIN((int64_t)(asize - ac), (int64_t)(m - bytes - w));
 	bytes = arc_evict_impl(arc_mfu, ARC_BUFC_METADATA, e);
 	total_evicted += bytes;
 	mfum -= bytes;
@@ -4320,14 +4321,14 @@ arc_evict(void)
 	/* Evict MRU data. */
 	wt -= m - total_evicted;
 	w = wt * (int64_t)(arc_pd >> 16) >> 16;
-	e = MIN((int64_t)(asize - arc_c), (int64_t)(mrud - w));
+	e = MIN((int64_t)(asize - ac), (int64_t)(mrud - w));
 	bytes = arc_evict_impl(arc_mru, ARC_BUFC_DATA, e);
 	total_evicted += bytes;
 	mrud -= bytes;
 	asize -= bytes;
 
 	/* Evict MFU data. */
-	e = asize - arc_c;
+	e = asize - ac;
 	bytes = arc_evict_impl(arc_mfu, ARC_BUFC_DATA, e);
 	mfud -= bytes;
 	total_evicted += bytes;