Hello. The diff in this email removes dead code in uvm_pmr_getpages().
This had bothered me for a bit because I had intuitively believed it wasn't
necessary, but had never bothered to fully convince myself. Since I have now,
I'll share the reasoning.
This diff concerns the following bit of code:
if (maxseg == 1 || count == 1) {
start_try = 2;
search[2] = count;
} else if (maxseg >= count && (flags & UVM_PLA_TRYCONTIG) == 0) {
start_try = 2;
search[2] = 1;
} else {
start_try = 0;
search[0] = count;
search[1] = pow2divide(count, maxseg);
search[2] = 1;
if ((flags & UVM_PLA_TRYCONTIG) == 0)
start_try = 1;
- if (search[1] >= search[0]) {
- search[1] = search[0];
- start_try = 1;
- }
if (search[2] >= search[start_try]) {
start_try = 2;
}
}
The lines indicated are the ones to be removed. The reason is that search[1]
will always be less than search[0] at this point in the code.
Proof: From pow2divide() we know that search[1] is the smallest power of 2 that
is >= 1 such that:
search[1]*maxseg >= count == search[0].
This means that if search[1] is > 1 that search[1]*maxseg/2 < search[0]. We
already know that maxseg is at least 2 because of the branch
if (maxseg == 1 || count == 1)
that would have been taken otherwise. Therefore, maxseg/2 is at least 1, so:
search[1] <= search[1]*maxseg/2 < search[0]
when search[1] is at least 2. If search[1] is 1, then search[1] < search[2]
because search[2] == count and count is at least 2 from the branch mentioned
previously. QED.
Index: uvm_pmemrange.c
===================================================================
RCS file: /cvs/src/sys/uvm/uvm_pmemrange.c,v
retrieving revision 1.63
diff -u -p -u -p -r1.63 uvm_pmemrange.c
--- uvm_pmemrange.c 10 Apr 2023 04:21:20 -0000 1.63
+++ uvm_pmemrange.c 10 Jun 2023 16:28:47 -0000
@@ -909,10 +909,6 @@ uvm_pmr_getpages(psize_t count, paddr_t
search[2] = 1;
if ((flags & UVM_PLA_TRYCONTIG) == 0)
start_try = 1;
- if (search[1] >= search[0]) {
- search[1] = search[0];
- start_try = 1;
- }
if (search[2] >= search[start_try]) {
start_try = 2;
}
