D36194.diff
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	Index: sys/kern/vfs_vnops.c
	===================================================================
	--- sys/kern/vfs_vnops.c
	+++ sys/kern/vfs_vnops.c
	@@ -3263,13 +3263,24 @@
	len = savlen = rounddown(len - rem, blksize) + rem;
	}

	- if (blksize <= 1)
	+ /*
	+ * _PC_MIN_HOLE_SIZE is expressing the alignment guarantees of the
	+ * holes, not necessarily what we should use as a block size.
	+ * If the result to too small (512 bytes or less), use the
	+ * filesystems preferred block size instead.
	+ * We enforce a lower bound of 4096 bytes to avoid copying in
	+ * microscopic chunks, and an upper bound of maxphys to limit
	+ * the size of the memory allocation for the temporary buffer
	+ * in a way that can scale up for large machines, but remain
	+ * reasonable on smaller 32bit platforms.
	+ */
	+ if (blksize <= 512)
	blksize = MAX(invp->v_mount->mnt_stat.f_iosize,
	outvp->v_mount->mnt_stat.f_iosize);
	if (blksize < 4096)
	blksize = 4096;
	- else if (blksize > 1024 * 1024)
	- blksize = 1024 * 1024;
	+ else if (blksize > maxphys)
	+ blksize = maxphys;
	dat = malloc(blksize, M_TEMP, M_WAITOK);

	/*
	@@ -3378,7 +3389,7 @@
	}

	xfer = blksize;
	- if (cantseek) {
	+ if (__predict_false(cantseek)) {
	/*
	* Set first xfer to end at a block boundary, so that
	* holes are more likely detected in the loop below via