Fix Linux available memory reporting to use MemAvailable instead of get_avphys_pages()

On Linux, get_avphys_pages() maps to sysconf(_SC_AVPHYS_PAGES) which returns
only completely unallocated pages (MemFree). The Linux kernel eagerly uses
spare physical RAM for disk caching (buff/cache). When active disk caching
fills memory, MemFree routinely hovers between 200-500 MB even when gigabytes
of memory are available.

This caused the viewer's memory budget logic to falsely report low system
memory (<256 MB), repeatedly triggering emergency texture downrezzing loops
and dynamic draw-distance halving.

Replace get_avphys_pages() in LLMemoryInfo::updateAvailableMemory() with a
fast, non-allocating single fread of MemAvailable: from /proc/meminfo
(available on Linux >= 3.14), with fallback to get_avphys_pages().

1 files changed, 27 insertions, 2 deletions

diff --git a/indra/llcommon/llsys.cpp b/indra/llcommon/llsys.cpp
index bc38b72004..8ebb45ca66 100644
--- a/indra/llcommon/llsys.cpp
+++ b/indra/llcommon/llsys.cpp
@@ -853,8 +853,33 @@ void LLMemoryInfo::updateAvailableMemory()
     }
 
 #elif LL_LINUX
-    U64 phys = U64(getpagesize()) * U64(get_avphys_pages());
-    LLMemory::sAvailPhysicalMemInKB = U64Bytes(phys);
+    bool found_available = false;
+    LLFILE* fp = LLFile::fopen(MEMINFO_FILE, LLFILE_MODE("rb"));
+    if (fp)
+    {
+        char buff[2048];
+        size_t nbytes = fread(buff, 1, sizeof(buff) - 1, fp);
+        buff[nbytes] = '\0';
+        fclose(fp);
+
+        char* memp = strstr(buff, "MemAvailable:");
+        if (memp)
+        {
+            unsigned long long mem_avail_kb = 0;
+            if (sscanf(memp, "MemAvailable: %llu", &mem_avail_kb) == 1)
+            {
+                LLMemory::sAvailPhysicalMemInKB = U32Kilobytes(mem_avail_kb);
+                found_available = true;
+            }
+        }
+    }
+
+    if (!found_available)
+    {
+        // Fallback for pre-3.14 kernels or container environments without MemAvailable
+        U64 phys = U64(getpagesize()) * U64(get_avphys_pages());
+        LLMemory::sAvailPhysicalMemInKB = U64Bytes(phys);
+    }
 #else
     //do not know how to collect available memory info for other systems.
     //leave it blank here for now.