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Tashkinov" Subject: mm: tmpfs: huge=always Kconfig help understates severe space amplification Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: quoted-printable X-Provags-ID: V03:K1:xPe759PgRxemlHDr55Fx082NlvzbE+Q1otpItHIcym3ZjYJ++Zn kZgmbtIF05SLsfsocZsjna0DWdPIacKvLbe+ghl3RzZZ6Xq45CI6Wlb5ZhE3Br0kURVWadz TGk5ukLRdKK/ys8rm8nIdTCOySWuZQmZt89jEXRgC4I/F48d7Wt7GdqCX64BUkob791e1Fm hNp4ZjW9xneUeKl1o4+eQ== X-Spam-Flag: NO UI-OutboundReport: notjunk:1;M01:P0:Nlp4ASWUcvY=;x26xLF1PeZhZYQMlPGuzCfCutpf 5Ee69RsKGymm1eTMv1Gi6CovyvAY0GKBuSuPPgGfAzrVN7VNZK1BBxoz85KxY1D5CvF//pqe8 R5ObrWcbW7qSwOvz/xaRybBbKHYIH/V66EOPx/ucRbn0+0JYvGcLXet6rREan4JbjQEAbgYXS MBSOWZQ3PGwra3e0ij4DuXvTBwcx4LCr0cUQXnl8IW+rWhVsW5lCTHcd5h8JUqQp3S/JLEle7 lB5h4d7ClfPfRJKGAvtD32yvzG99U0/ZATy+Ro2ertCRxXF3MS1FGj39REe0kA+t8LIYJgNT1 nTgRrff9s94owrrwQQkiPR96jllHn8gb9XLkzAzOLmNqeUeC5dM1ZRRhHsYfoDzrvvUI75fSx g526jKOKjg4l08iR1mdIX5givQrybT1OxiCZ8PKCbtIHnycfLNB5Wo2nCr7kBFrmaeXnYqg/F 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The help text currently says: Always attempt to allocate hugepage for tmpfs mount, can increase the memory footprint of applications without a guaranteed benefit but it will work automatically for all applications. I believe this substantially understates the potential consequences of=20 selecting this option, particularly on general-purpose systems where=20 /tmp is mounted as tmpfs. I recently encountered the following situation while extracting a Fedora= =20 filesystem tree under /tmp on an x86-64 system running Linux 7.3. The tmpfs mount had huge=3Dalways enabled. $ df -hT /tmp/Fedora Filesystem Type Size Used Avail Use% tmpfs tmpfs 48G 47G 925M 99% $ du -sh /tmp/Fedora/usr 45G /tmp/Fedora/usr $ du -sh --apparent-size /tmp/Fedora/usr 1.4G /tmp/Fedora/usr Even more strikingly: $ du -sh /tmp/Fedora/usr/share/man/man3 7.5G /tmp/Fedora/usr/share/man/man3 $ du -sh --apparent-size /tmp/Fedora/usr/share/man/man3 7.8M /tmp/Fedora/usr/share/man/man3 Thus, a directory containing just 7.8 MiB of apparent file data occupied= =20 approximately 7.5 GiB of allocated space. The overall tree occupied approximately 45 GiB despite containing only=20 1.4 GiB of apparent file data. This is consistent with severe allocation amplification caused by=20 transparent huge pages, although I have not yet established the exact=20 breakdown of huge-page allocations and other tmpfs overhead. The problem with the existing Kconfig help is not that it fails to=20 mention additional memory consumption. It does mention that. The problem is that "can increase the memory footprint" gives no=20 indication of the magnitude involved, nor does it explain the workloads=20 most likely to suffer. A kernel builder selecting "always" may reasonably expect better=20 performance at the cost of somewhat higher memory consumption. The help=20 does not adequately warn that workloads involving many small files can=20 experience extreme space amplification, potentially exhausting a tmpfs=20 filesystem containing relatively little actual data. There is also an important distinction between enabling transparent huge= =20 pages for selected workloads and making huge-page allocation the default= =20 policy for all tmpfs mounts. I suggest expanding the Kconfig help text to explicitly mention: - The potential for very large allocation overhead with small files. - The risk of exhausting tmpfs space far sooner than apparent file sizes= =20 would suggest. - The fact that selecting "always" is not a universal performance=20 optimisation. - The availability of "never", "within_size", and "advise" as=20 alternative policies. For example, the help might say: Always attempt hugepage allocation for tmpfs mounts. WARNING: This policy can substantially increase memory and tmpfs space consumption, particularly for workloads creating large numbers of small files. Allocated space may greatly exceed apparent file sizes, potentially exhausting tmpfs capacity even when relatively little file data is stored. This setting does not guarantee improved performance and should only be selected when its memory overhead is understood and acceptable for the intended workload. Consider "never", "within_size", or "advise" for general-purpose systems. I appreciate that the upstream default remains "never". My concern is=20 specifically that the help text does not adequately communicate the=20 consequences of explicitly selecting "always". A more prominent warning would help kernel builders make an informed=20 choice and avoid potentially severe memory consumption surprises. Regards, Artem